Create contributing.md

Signed-off-by: Fabiana Campanari <[email protected]>

.github/contributing.md

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+# Contributing
+
+## Security / Disclosure
+
+If you find any bug with Renovate that may be a security problem, then e-mail us at: [[email protected]](mailto:[email protected]).
+This way we can evaluate the bug and hopefully fix it before it gets abused.
+Please give us enough time to investigate the bug before you report it anywhere else.
+
+Please do not create GitHub issues for security-related doubts or problems.