Reword disabling signature section of README

Make it clear that this shouldn't be necessary, even for proprietary
repos.

Change-Id: Ica4bb5d8d38e17d2047a4cc384e6afbef45b4dcb
Reviewed-on: http://gerrit.causes.com/37396
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>
Update overcommit-hook before hook run if changed

Previously, if upgrading Overcommit would result in the master hook
being changed, the user would have to manually run `overcommit
--install` in order to update the hooks.

Make this a little bit easier by automatically updating the master hook
if it has changed.

One trick to get this to work is that if the hook is changed and we
update it, we need to `exec` in order to replace the current process
with a new one executing the new master hook, otherwise the old file
will still be run.

Change-Id: I6746e6f25cf624c0da9d91153ba1f91b9a9b8e75
Reviewed-on: http://gerrit.causes.com/37388
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>