Add note about makeprg and sandbox

Changing `makeprg` is not possible in sandbox mode. I added a corresponding notice to the help text.

From VIM's help on `makeprg`:

> This option cannot be set from a |modeline| or in the |sandbox|, for
> security reasons.

I don't know if this was introduced in a specific VIM version, so here is the version I'm using:
> VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Aug 12 2013 00:23:30)