Mention that users might need to install GNU Diff.
Mention that users might need to install GNU Diff.

For some SCMs, users (particularly on windows) will need to install GNU diff
themselves. This change mentions this in the installation docs and links to an
installer. While I was in here I also mentioned Mac OS X alongside Linux.

Testing done: Built html docs and checked that links worked.

Fixes bug 2058.

beol/rbtools,davidt/rbtools,davidt/rbtools,reviewboard/rbtools,datjwu/rbtools,haosdent/rbtools,beol/rbtools,reviewboard/rbtools,beol/rbtools,reviewboard/rbtools,halvorlu/rbtools,datjwu/rbtools,datjwu/rbtools,halvorlu/rbtools,haosdent/rbtools,haosdent/rbtools,1tush/rbtools,davidt/rbtools,halvorlu/rbtools