Update Readme.md for Open Collective

Hi, I'm making updates for Open Collective. Either you or a supporter signed this repo up for Open Collective. This pull request adds backers and sponsors from your Open Collective https://opencollective.com/leetcode-280❤️

It adds two badges at the top to show the latest number of backers and sponsors. It also adds placeholders so that the avatar/logo of new backers/sponsors can automatically be shown without having to update your README.md. [more info](https://github.com/opencollective/opencollective/wiki/Github-banner). See how it looks on this [repo](https://github.com/apex/apex#backers).

You already have a manual contributor image, but I added the automated Open Collective contributor image anyways. 

You can also add a postinstall script to let people know after npm|yarn install that you are welcoming donations (optional). [More info](https://github.com/OpenCollective/opencollective-cli)
You can also add a "Donate" button to your website and automatically show your backers and sponsors there with our widgets. Have a look here: https://opencollective.com/widgets

P.S: As with any pull request, feel free to comment or suggest changes. The only thing "required" are the placeholders on the README because we believe it's important to acknowledge the people in your community that are contributing (financially or with code!).

Thank you for your great contribution to the open source community. You are awesome! 🙌
And welcome to the open collective community! 😊

Come chat with us in the #opensource channel on https://slack.opencollective.com - great place to ask questions and share best practices with other open source sustainers!

Author: monkeywithacupcake

README.md

@@ -8,6 +8,8 @@
   <a href="https://github.com/doocs/leetcode/issues"><img src="https://img.shields.io/github/issues/doocs/leetcode.svg" alt="issues"></a>
   <a href="https://github.com/doocs/leetcode/stargazers"><img src="https://img.shields.io/github/stars/doocs/leetcode.svg" alt="stars"></a>
   <a href="https://github.com/doocs/leetcode/network/members"><img src="https://img.shields.io/github/forks/doocs/leetcode.svg" alt="forks"></a>
+  <a href="#backers" alt="sponsors on Open Collective"><img src="https://opencollective.com/leetcode-280/backers/badge.svg" /></a> 
+  <a href="#sponsors" alt="Sponsors on Open Collective"><img src="https://opencollective.com/leetcode-280/sponsors/badge.svg" /></a>
 </p>
 
 ## Introduction
@@ -36,4 +38,32 @@ I'm looking for long-term contributors/partners to this repo! Send me [PRs](http
 | <center> [<img src="https://avatars3.githubusercontent.com/u/32598987?v=4" width="80px;"/>](https://github.com/xiapengchng) </center> | <center> [<img src="https://avatars3.githubusercontent.com/u/37660444?v=4" width="80px;"/>](https://github.com/Mcnwork2018) </center> | <center> [<img src="https://avatars3.githubusercontent.com/u/22535595?v=4" width="80px;"/>](https://github.com/bluesword12350) </center> | <center> [<img src="https://avatars3.githubusercontent.com/u/39827514?v=4" width="80px;"/>](https://github.com/ashwek) </center> | <center> [<img src="https://avatars3.githubusercontent.com/u/25475525?v=4" width="80px;"/>](https://github.com/Mrtj2016) </center> | <center> [<img src="https://avatars3.githubusercontent.com/u/19249839?v=4" width="80px;"/>](https://github.com/biubiubiubiubiubiubiu) </center> | <center> [<img src="https://avatars3.githubusercontent.com/u/31436272?v=4" width="80px;"/>](https://github.com/Neil94n) </center> | <center> [<img src="https://avatars3.githubusercontent.com/u/9976942?v=4" width="80px;"/>](https://github.com/LZHD) </center> |
 | <center> [<img src="https://avatars3.githubusercontent.com/u/31801502?v=4" width="80px;"/>](https://github.com/Wushiyii) </center> | | | | | | | |
 
-<!-- ALL-CONTRIBUTORS-LIST:END -->
+<!-- ALL-CONTRIBUTORS-LIST:END -->
+
+## Contributors
+
+This project exists thanks to all the people who contribute. 
+<a href="https://github.com/doocs/leetcode/graphs/contributors"><img src="https://opencollective.com/leetcode-280/contributors.svg?width=890&button=false" /></a>
+
+
+## Backers
+
+Thank you to all our backers! 🙏 [[Become a backer](https://opencollective.com/leetcode-280#backer)]
+
+<a href="https://opencollective.com/leetcode-280#backers" target="_blank"><img src="https://opencollective.com/leetcode-280/backers.svg?width=890"></a>
+
+
+## Sponsors
+
+Support this project by becoming a sponsor. Your logo will show up here with a link to your website. [[Become a sponsor](https://opencollective.com/leetcode-280#sponsor)]
+
+<a href="https://opencollective.com/leetcode-280/sponsor/0/website" target="_blank"><img src="https://opencollective.com/leetcode-280/sponsor/0/avatar.svg"></a>
+<a href="https://opencollective.com/leetcode-280/sponsor/1/website" target="_blank"><img src="https://opencollective.com/leetcode-280/sponsor/1/avatar.svg"></a>
+<a href="https://opencollective.com/leetcode-280/sponsor/2/website" target="_blank"><img src="https://opencollective.com/leetcode-280/sponsor/2/avatar.svg"></a>
+<a href="https://opencollective.com/leetcode-280/sponsor/3/website" target="_blank"><img src="https://opencollective.com/leetcode-280/sponsor/3/avatar.svg"></a>
+<a href="https://opencollective.com/leetcode-280/sponsor/4/website" target="_blank"><img src="https://opencollective.com/leetcode-280/sponsor/4/avatar.svg"></a>
+<a href="https://opencollective.com/leetcode-280/sponsor/5/website" target="_blank"><img src="https://opencollective.com/leetcode-280/sponsor/5/avatar.svg"></a>
+<a href="https://opencollective.com/leetcode-280/sponsor/6/website" target="_blank"><img src="https://opencollective.com/leetcode-280/sponsor/6/avatar.svg"></a>
+<a href="https://opencollective.com/leetcode-280/sponsor/7/website" target="_blank"><img src="https://opencollective.com/leetcode-280/sponsor/7/avatar.svg"></a>
+<a href="https://opencollective.com/leetcode-280/sponsor/8/website" target="_blank"><img src="https://opencollective.com/leetcode-280/sponsor/8/avatar.svg"></a>
+<a href="https://opencollective.com/leetcode-280/sponsor/9/website" target="_blank"><img src="https://opencollective.com/leetcode-280/sponsor/9/avatar.svg"></a>