Add security meta-doc [skip ci]

.github/SECURITY.md

+# Security Policy
+
+Please use the [security issue form](https://www.yiiframework.com/security) to report to us any security issue you find in Yii.
+DO NOT use the issue tracker or discuss it in the public forum as it will cause more damage than help.
+
+Please note that as a non-commerial OpenSource project we are not able to pay bounties at the moment.