Merge pull request #669 from WordPress-Coding-Standards/feature/updat…

…e-readme

Update readme section on PHPCompatibility

README.md

@@ -148,6 +148,11 @@ The [PHPCompatibility](https://github.com/wimg/PHPCompatibility) ruleset comes h
 The [PHPCompatibility](https://github.com/wimg/PHPCompatibility) sniffs are designed to analyse your code for cross-PHP version compatibility.
 Install it as a separate ruleset and either run it separately against your code or add it to your custom ruleset.
 
+Whichever way you run it, do make sure you set the `testVersion` to run the sniffs against. The `testVersion` determines for which PHP versions you will received compatibility information. The recommended setting for this at this moment is  `5.2-7.1` to support the same PHP versions as WordPress Core supports.
+
+For more information about setting the `testVersion`, see:
+* [PHPCompatibility: Using the compatibility sniffs](https://github.com/wimg/PHPCompatibility#using-the-compatibility-sniffs)
+* [PHPCompatibility: Using a custom ruleset](https://github.com/wimg/PHPCompatibility#using-a-custom-ruleset)
 
 ## Fixing errors or whitelisting them