Revise installation instructions

From OpenFOAMWiki

Hi Bruno, Thanks for your message. Please do include the mirror links. Thank you very much!! Best regards, Dominik

Dchrist (talk)12:45, 13 May 2014

I've added the mirror information and since the Arch Linux related page has been updated, I've rehashed the old idea of using sub-pages for system-specific instructions.

Wyldckat (talk)15:23, 15 June 2014