Add a SECURITY.md

Add a SECURITY.md

Move content from the Security Process[1] and Security Exceptions[2]
wiki documents into the repository so that it is in a standard place for
analysis tools to look for the glibc security policy.

This is a more or less verbatim port of the wiki document with some
restructuring for a more coherent layout since the two pages are now
merged.  There should be no change in messaging in this commit.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>