[Chore] Add Contributing section to README

[daveallie]

As a contributor, it's difficult to understand what you'd like me to do in order to be able to submit pull requests to this gem.

The PR templates mentions linters, security checks, and tests, but does not explain how to run or validate any of these things.

A contributing section in the README would be mighty helpful in explaining the few steps required for checking out the codebase and validating that changes are in-line with the requirements of the gem.