Add `Practical guide to getting started`

[franklin-feingold]

This PR seeks to add a Practical guide to getting started document. The purpose of this document is to introduce the key terms for navigating our GitHub organization and the GitHub process for submitting their contribution (fork, branch, pull request). I think it is beneficial to have a brief guide for introducing our community to our most common processes. This document also complements the detail provided in the contributing guide.

[poldrack]

I might make it a bit clearer that this is a guide for potential contributors

[franklin-feingold]

sounds good! I made the filename more clear and rephrased the introduction framing to address our potential contributor