DOC: previously supported OS versions installation hints

Add to the “Installation Prerequisites” section of TXM 0.7.8 download page a link to the “Previously supported OS versions installation hints” FAQ section: https://groupes.renater.fr/wiki/txm-users/public/faq#comment_verifier_que_txm_s_est_installe_correctement

FAQ section content

For Ubuntu 14.04:

- install TXM with dpkg and the —force-all option

- install R and the necessary packages independently

For Windows XP:

- …

For Mac OS X 10.8:

- …

(from redmine: issue id 2232, created on 2017/08/01 by Matthieu Decorde)