SECURITY.md: Add instructions for reporting security issues

SECURITY.md

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+How to Report a Potential Vulnerability?
+========================================
+
+If you would like to report a public issue (for example, one with a released
+CVE number), please report it via GitHub issues:
+
+  https://github.com/agherzan/meta-raspberrypi/issues/new/choose
+
+If you are dealing with a not-yet released or urgent issue, please send a
+message to one of the maintainers listed in the [README.md](https://github.com/agherzan/meta-raspberrypi/blob/master/README.md).  Include as many
+details as possible:
+  - the layer or software module affected
+  - the recipe and its version
+  - any example code, if available
+
+Branches maintained with security fixes
+---------------------------------------
+
+See https://wiki.yoctoproject.org/wiki/Releases for the list of current
+releases.  We only accept patches for the LTS releases and the master branch.