Documentation for Hooks

[Chris Rees]

Hi all,

If no-one has any objections, I'm going to commit the patch at [1] to
the README, which:

- includes an example of uses of Hooks to restrict the number of CPUs
while building (from itetcu)

- Corrects the error in instructions on removing a Hook (from wxs).

I'm asking for review because I've not personally used Hooks, so
please would someone double-check?

Chris

[1] http://www.bayofrum.net/~crees/patches/tinderweb-Hooks.diff

Rendered at http://www.bayofrum.net/~crees/tinderweb/README.html#USING-HOOKS