Add basic contributing info. Still need to add for pull requests, etc.

CONTRIBUTING.md

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+Welcome to firejail, and thank you for your interest in contributing!
+
+# Opening an issue:
+We welcome issues, whether to ask a question, provide information, request a new profile or
+feature, or to report a suspected bug or problem.
+
+If you want to request a program profile that we don't already have, please add a comment in
+our dedicated issue, #1139.
+
+When submitting a bug report, please provide the following information so that
+we can handle the report more easily:
+ - firejail version. If you're not sure, open a terminal and type `firejail --version`.
+ - Linux distribution (so that we can try to reproduce it, if necessary).
+ - If you know that the problem did not exist in an earlier version of firejail, please mention it.
+ - If you are reporting that a program does not work with firejail, please also run firejail with
+ the `--noprofile` argument.
+ For example, if `firejail firefox` does not work, please also run `firejail --noprofile firefox` and
+ let us know if it runs correctly or not.
+
+We take security bugs very seriously. If you believe you have found one, please report it by
+emailing us at [email protected]