Add note about macOS notarization

[TheSpydog]

For the sake of macOS developers who aren't totally comfortable telling their users to just disable Gatekeeper, it may be worth adding @nickgravelyn's macOS notarization guide to the README. (It was written for FNA games in particular, but there's nothing specific to FNA included in there.)

Thoughts? cc: @afrodynamics

[flibitijibibo]

The results of this will go into the FNA wiki as well. As long as we have at least two separate examples of this working we can document this officially.

[afrodynamics]

The README there looks good to me. I used the same codesign parameters, though I've yet to finish notarizing and shipping an actual build. Once a few more games go through the full process, I think it'd be a great addition to the wiki.