Adds local build instructions to README documentation

This is a quick update to the "Contributing" section of the README that adds some instructions on how to test the `scss-lint` binary locally after making a change to your fork or clone.

I was playing around with a local clone of SCSS-Lint and wanted to try using my version of the gem but couldn't find any suggestions on how to build the binary in the project documentation. Thought it might be useful to include a small guide in the documentation to get any potential contributors up and running.

If you have any changes you'd like made to the phrasing or instructions let me know!