Add additional contributing documentation on how the directory structure is laid out

[adam2392]

The general directory structure is:

• docs/ or doc/ hosting documentation that gets rendered into a webpage, which then you can see online
• mne-hfo/ is the directory with all the actual code
• .circleci/ hosts testing code setup to run things on CircleCI (a cloud service company for testing code continuously)
• .github/ hosts Github related code like templates for GH issues and also testing code setup for GH's cloud service for testing code
• tests/ is the directory with all the unit and integration tests that test various pieces of the mne-hfo code.