Create CONTRIBUTING.md

Author: dnozay

CONTRIBUTING.md

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+# Please take a moment to review the following documents:
+* https://wiki.jenkins-ci.org/display/JENKINS/Beginners+Guide+to+Contributing
+* https://wiki.jenkins-ci.org/display/JENKINS/Extend+Jenkins
+
+# For security issues:
+* https://wiki.jenkins-ci.org/display/SECURITY/SECURITY+issues+in+plugins
+* in short, do not post information publicly (disclosure) until a fix made available publicly.
+
+# Jenkins is about testing:
+* please add tests to cover each new feature; pull requests may be rejected.
+* please architect the code so it is easier to test.