Including documentation for setting up GitHub application and Gatekeeper

**Is your feature request related to a problem? Please describe.**

The current CATcher documentation is relatively sparse on how to setup the full application for dev mode. It took me a while to realize what was all required.

**Describe the solution you'd like**

The key points to note are:

1. Setting up a GitHub application with a custom client id
2. Setting up Gatekeeper locally
3. Modifying/adding an environment file with the custom client id, gatekeeper redirect URL, and client data URL

**Describe alternatives you've considered**

Alternatively, dockerize both CATcher and Gatekeeper to launch on 2 separate ports but so they are all bundled for dev purposes. This is mostly because the environment is hardcoded to a few values which may not be entirely accurate for dev.

**Additional context**

N/A

Start with CATcher's current developer setup documentation and inspect how the environment file is loaded. Document the required custom GitHub application client ID, local Gatekeeper setup, redirect URL, and client data URL; done means a newcomer can follow the steps to run CATcher in dev mode.

Written by the indexing model from the issue text.