[riot-notifications] [RIOT-OS/RIOT] SECURITY.md: Add initial security policy (#15516)

[MrKevinWeiss]

### Contribution description

RIOT should have a defined policy on how to handle security released bugs.

After a small survey of other OSes, this seems to fit best with RIOT.

You can view, comment on, or merge this pull request online at:

  https://github.com/RIOT-OS/RIOT/pull/15516

-- Commit Summary --

  * SECURITY.md: Add initial security policy

-- File Changes --

    A SECURITY.md (19)

-- Patch Links --

https://github.com/RIOT-OS/RIOT/pull/15516.patch
https://github.com/RIOT-OS/RIOT/pull/15516.diff

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