Install document missing PATH handling

Created by: mfn

https://github.com/gitlabhq/gitlabhq/wiki/Gitolite

writes

# install gitolite
sudo -u git -H /home/git/gitolite/src/gl-system-install

On my system I get:

$ sudo -u git -H /home/git/gitolite/src/gl-system-install
using default values for EUID=102:
/home/git/bin /home/git/share/gitolite/conf /home/git/share/gitolite/hooks

                ***** WARNING *****
gl-setup is not in your $PATH.

Since gl-setup MUST be run from the PATH (and not as src/gl-setup or such),
you must fix this before running gl-setup.  The simplest way is to add

    PATH=/home/git/bin:$PATH

to the end of your bashrc or similar file.  You can even simply run that
command manually each time you log in and want to run a gitolite command.

Run /home/git/gitolite/src/gl-system-install -h for a detailed usage message.

Using the shell trick just used in the line below solves this:

sudo -u git -H sh -c "PATH=/home/git/bin:$PATH; /home/git/gitolite/src/gl-system-install"