Update testing instructions

Author: thinkingserious

CONTRIBUTING.md

@@ -129,6 +129,22 @@ All test files are in the [`tests`](https://github.com/sendgrid/sendgrid-java/tr
 
 For the purposes of contributing to this repo, please update the [`SendGridTest.java`](https://github.com/sendgrid/sendgrid-java/tree/master/src/test/java/com/sendgrid/SendGridTest.java) file with unit tests as you modify the code.
 
+1. Download [prism](https://stoplight.io/prism/) for your platform ([Mac OS X](https://github.com/stoplightio/prism/releases/download/v0.1.5/prism_darwin_amd64)) and save the binary to the sendgrid-ruby directory (or any directory you would like. The sendgrid-ruby directory is chosen mostly for convenience.)
+
+1. Add execute permissions
+
+```bash
+chmod +x prism
+```
+
+1. In a separate terminal, cd into the directory you chose for prism and start the sendgrid local server which the tests will use.
+
+```bash
+./prism run --mock --list --spec https://raw.githubusercontent.com/sendgrid/sendgrid-oai/master/oai_stoplight.json
+```
+
+1. Now you can run the test suite from the root of the project
+
 ```bash
 ./gradlew test -i
 ```