Add checklist for the FDA General Software Guidance document

[johndgiese]

We will slowly add checklists for a few different FDA Guidance documents. Requirements in guidance documents will likely be less clearly defined than in ISO standards.

It may also be useful to extend the checklist format a bit so we can keep the documents simple. In particular, it may be nice to include lines like:

FDASG:4.2==62304:5.1.3 Optional comment goes here

to indicate that meeting this part of 62304 also meets the related guidance requirement.