Add Contributing File

[seanmarcia]

Summary

We don't have a Contributing.md file in this app! We need to add one. Ruby for Good has a template that can be found here that should be used as the core of this PR. Other things to add would be any specific software requirements, database requirements, and so on. We should have the majority of the relevant information available throughout the Readme, wiki, and other files we just need to put it together.

Things to Consider

A lot of inspiration can likely be derived from the various RfG apps.

Criteria for Completion

• Create a Contributing.md file for human-essentials