X-Debbugs-Cc: philipk@posteo.net

 From the discussion in bug#64985, Eli mentioned that .elpaignore wasn't 
documented. (It turns out it's mentioned briefly in the GNU ELPA README, 
but that's the only place I could find.) Here's a small patch to 
document this in the Package section of the Emacs Lisp manual.

This is just a first pass at documenting this feature, so I'm happy to 
add further details if anyone thinks it's warranted.