Add documentation explaining mock directory and its contents

[harrisonCassar]

It's amazing that we already have developed source code (located in the mock directory) that essentially stubs out the hardware aspect of our flight loop! Let's document this, potentially in a nice README complemented with some other markdown documents explaining potentially:

• How it works
• Why we decided to build this infrastructure
• How to use the mock in runs of the SPARTAN FSW (likely just the options/steps needed to build SPARTAN FSW with it)
• Limitations of using the mock (in terms of accuracy/validity of returned data when reading from them). This is primarily to ensure that those utilizing the mock for development recognizes its limitations for testing/confirming validity of FSW implementation.
• How to make changes to mock contents (this may well be integrated into the master README being developed in Add master README #10, but let's still develop this here).
• Other important things that should be documented!

For now, besides the README (which should be in the mock directory), let's just place these documents within a new sub-directory mock/docs. Later, either with or after Issue #12 is addressed, we can figure out if we need to re-locate these documents.