github: add contributing doc

.github/CONTRIBUTING.md

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+Contributing to Suricata
+========================
+
+We're happily taking patches and other contributions. The process is documented at https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Contributing Please have a look at this document before submitting.
+
+Contribution Agreement
+----------------------
+
+Before accepting your pull requests we need you or your organization to sign our contribution agreement.
+
+We do this to keep the ownership of Suricata in one hand: the Open Information Security Foundation. See https://suricata-ids.org/about/open-source/ and https://suricata-ids.org/about/contribution-agreement/
+
+Contribution Process
+--------------------
+
+Suricata is a complex piece of software dealing with mostly untrusted input. Mishandling this input will have serious consequences:
+
+* in IPS mode a crash may knock a network offline;
+* in passive mode a compromise of the IDS may lead to loss of critical and confidential data;
+* missed detection may lead to undetected compromise of the network.
+
+In other words, we think the stakes are pretty high, especially since in many common cases the IDS/IPS will be directly reachable by an attacker.
+
+For this reason, we have developed a QA process that is quite extensive. A consequence is that contributing to Suricata can be a somewhat lengthy process.
+
+On a high level, the steps are:
+
+1. Travis-CI based build & unit testing. This runs automatically when a pull request is made.
+
+2. Review by devs from the team and community
+
+3. QA runs trigged by the team
+
+Questions
+---------
+
+If you have questions about contributing, please contact us via https://suricata-ids.org/support/
+