mention exim workaround in installation doc
Wording from https://github.com/gitlabhq/gitlabhq/issues/4866#issuecomment-118912798
Also tested on gitlab.pxq.in
Wording from https://github.com/gitlabhq/gitlabhq/issues/4866#issuecomment-118912798
Also tested on gitlab.pxq.in