Add some details about setup to the docs?

[maelle]

Thanks for this neat package!

I was starting to use it and convinced I had set up gmailr correctly since I recently used it... I had forgotten to run gmailr::gm_auth_configure() 😬 I was wondering:

• whether mailmerge should catch and render gmailr error message? I didn't see it, I added a browser() inside mailmerge.
• whether the docs could refer to the docs of gmailr reg setup so that my stupid mistake would be harder to make?

[andrie]

I've fixed this in #4