Add note about no-op for global context action

[fidgetingbits]

This was something I was looking for awhile back as well, and it just came up on slack recently, so likely worth documenting:

What is the recommended way to define an action that is a no-op when in global context, but does something in a specific context? I believe Talon has some no-op checking code so that it can complain if you call an action that you forgot to implement in a given context cc/ Andreas Arvidsson

Response from Andreas

Personally I'm a fan of actions.skip(). I think the intent there is very clear both to a human reader and to Talon.