Add doc for directories to likely mark so that IDEA project can compile

Using IntelliJ IDEA shows that one needs to mark some buck-out/
annotation directories as Generated Sources Root, in order for the
Gerrit project to compile. That can be done through the Project view
contextual menu, upon selecting each directory. Directories to mark are
listed by this changed developer doc.

Change-Id: I03fe7cb21fa251656130a5a90079ed77703f60cb
1 file changed