Add new CONTRIBUTING.md
Various and sundry text updates to CONTRIBUTING.md

Namely, all sentences should end with periods. Other light changes.
Please cover the rest of my comments mentioned on commit 30fec1af (or
argue with them) then PR away!
Prevent jsobfu from generating duplicate/reserved tokens.

I got an error from a script that tried to 'set void = 1'.
Add future reserved words.

Gotta stay ahead of the game.
Merge remote-tracking branch 'origin/pr/4' into feature/contributing
Make random_var_name public, add specs for it.
Remove unneeded quotes.
Add presence spec.
Dropped a space.
Add Rex::RandomIdentifierGenerator.
Kill extra comma.
Oops, $ and _ need to be in the spec.

* Repeats the random check 20 times for each spec.
Use be instead of eq.
Add note about reporting vulns
Add an example of a good PR
Add missing period
Actually use the argument
Land #3062, unused arg fix for vmware_mount
Land #3058 - Prevent jsobfu from generating reserved js keywords
Add more examples of good contributions