Document sudo policy for gitfs post-recieve hook

[amendlik]

What does this PR do?

Update documentation for creating a git hook to update gitfs.

What issues does this PR fix or reference?

Because the hook is run as the user performing the push,
salt-call will typically fail.

Previous Behavior

Git pushes not done by root would fail to run the git
post-receive hook

New Behavior

Updates the documentation to use sudo for salt-call, and provides
a sudo policy that permits any user to run the post-recieve hook.

Tests written?

No

[terminalmage]

I think we need more information on why we are telling people to use the sudoers change. Also, salt may not be running as root, so we should clarify a few points:

1. This sudoers change needs to be made in cases where the user on the git server initiating the push is not the same as the user under which salt is running.
2. The policy should instead be ALL ALL=(root) NOPASSWD: SALT_GIT_HOOK to be more restrictive.
3. A clarification should be added noting that where the root user is mentioned (in the hook itself, and in point number 2 above), root should be the user under which salt is running (which in the vast majority of cases will be root, but in some cases may be a different user).

[amendlik]

Thank you for the feedback, @terminalmage. I agree with everything you've said and will make those changes.

[terminalmage]

Awesome, thanks! And thanks for contributing.