Added information to the README about docker-seleniarm community repo

[jamesmortensen]

Added information to the README about the location of the multi-arch container images GitHub repo and Docker Hub organization.

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Description

Updated the README to do two things:

• Point to the location of the stable Selenium x86_64 container images on Docker Hub.
• Point to the location of the docker-seleniarm community repository for mult-arch images and the images on Docker Hub.

Motivation and Context

• See armhf docker image of Selenium #1076

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [ X ] I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[CLAassistant]

All committers have signed the CLA.

[diemol]

Thank you, @jamesmortensen!