HACKING: Don't recommend Apache 2.0 for contributions

Since waffle changed from Apache 2.0 to BSD in
367378863bcc6801f5ae5ad786c09d39e0890370, we should avoid accepting
contributions under that license for the same reasons.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>