lib/fuzzing: Add oss-fuzz info to README.md

lib/fuzzing: Add oss-fuzz info to README.md

Note that Samba has not been accepted yet, but will be soon once some requirements
are addressed per:

https://github.com/google/oss-fuzz/pull/2993

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Uri Simchoni <uri@samba.org>
Autobuild-User(master): Douglas Bagnall <dbagnall@samba.org>
Autobuild-Date(master): Thu Nov 21 00:45:33 UTC 2019 on sn-devel-184