Update documentation

[sonofmun]

From Lenny Muellner:

Because the documentation mentions installing it locally first, it appears that one must install it locally. Lenny's suggestion:

So the first thing that you see is how to install it — but actually, you don’t need to do that if you are going to use Travis to run Hooktest — so maybe the better thing would be to start with, there are two ways to use Hooktest; one is to install and run it locally on your own system, the other is to run it on Travis. Seems self-evident, but it isn’t, since it looks as though you have to do the install before you do anything. See what I mean?