R1:f0618d54e8b5

Installation Guide: mention that git 2.5.0 (2015) is needed server-side

Summary:
Note that the git version 2.5.0 is surely already assumed
as required as implicit fact, so it's just better to clarify
this in the documentation to prevent weird situations and
unclear related exceptions.

Note that if you have seen this Phabricator/Phorge error
in your installation:

This path was a submodule at R123:5378198ea123asdlol

This is probably that kind of weird errors that will be
fixed just updating your git…