Add ./SECURITY.md (#1227)
Add ./SECURITY.md (#1227)

There's an emerging convention that ./SECURITY.md has some
information about security.  Let's support that convention
by adding that file, with contents that point interested users
to show where they should go.

A lot of projects that support this convention.  You can see a list here:
https://www.google.com/search?q=security.md+site%3Agithub.com
Examples include:
* JavaScript standard style: https://github.com/standard/standard
* Electron: https://github.com/electron/electron/blob/master/SECURITY.md
* Discourse: https://github.com/discourse/discourse/blob/master/docs/SECURITY.md
* Tensorflow: https://github.com/tensorflow/tensorflow/blob/master/SECURITY.md

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>
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