Document all targets in project.Makefile

[eecavanna]

Running $ make {some_target} seems to me to be something contributors to this repo do often.

I don't know what the various make targets do. I recommend adding a comment above each one, that says what it does.

[eecavanna]

I found one approach for documenting a Makefile's targets:
https://stackoverflow.com/a/47107132

The approach seems a little fragile to me (e.g. leading to people wondering "why's it working on line X, but not on line Y, as they look similar to me...?"), but it would give us at least the following two benefits:

• The ability to display the help for each command, all at once
• A standard way to document the targets