Add security policy.

Change-Id: I3819028fe680b33208878b5f6b568a281fa4d8aa

SECURITY.md

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+# Security Policy
+
+libavifinfo uses [Chromium's Security process](https://www.chromium.org/Home/chromium-security/).
+In order to report a libavifinfo security bug, please follow these steps:
+
+1. Check [Chromium's guidelines for security bugs](https://www.chromium.org/Home/chromium-security/reporting-security-bugs/)
+2. Create a [new bug](https://bugs.chromium.org/p/chromium/issues/entry?template=Security%20Bug) and add the component "Internals>Images>Codecs"