Document need for 'roottree' and 'file' settings in config

In order for Gerrit to pick up a custom gitweb config, it is now [1]
also required to define 'roottree' and 'file' patterns. (Otherwise
gerrit will not show gitweb links).

Branch-network only cares about projects, so we just copy/paste the
default of the other patterns.

[1] See Gerrit core's commit

  79d4ebec494ff5bb0b837e689452db9bc39af5fe

Change-Id: I47401760f301df6a80a38841c33405ba3980a3ac