add CONTRIBUTING.md

GitHub displays this file to users opening new Issues.
It is extracted from our[GitHub Workflow](https://github.com/ipython/ipython/wiki/Dev:-GitHub-workflow),
and links there for further details.

closes  #2411

CONTRIBUTING.md

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+## Opening an Issue
+
+When opening a new Issue, please take the following steps:
+
+1. Search GitHub and/or Google for your issue to avoid duplicate reports.
+   Keyword searches for your error messages are most helpful.
+2. If possible, try updating to master and reproducing your issue,
+   because we may have already fixed it.
+3. Try to include a minimal reproducible test case
+4. Include relevant system information.  Start with the output of:
+
+        python -c "import IPython; print(IPython.sys_info())"
+
+   And include any relevant package versions, depending on the issue,
+   such as matplotlib, numpy, Qt, Qt bindings (PyQt/PySide), tornado, web browser, etc.
+
+## Pull Requests
+
+Some guidelines on contributing to IPython:
+
+* All work is submitted via Pull Requests.
+* Pull Requests can be submitted as soon as there is code worth discussing.
+  Pull Requests track the branch, so you can continue to work after the PR is submitted.
+  Review and discussion can begin well before the work is complete,
+  and the more discussion the better.
+  The worst case is that the PR is closed.
+* Pull Requests should generally be made against master
+* Pull Requests should be tested, if feasible:
+    - bugfixes should include regression tests
+    - new behavior should at least get minimal exercise
+
+[Travis](http://travis-ci.org/#!/ipython/ipython) does a pretty good job testing IPython and Pull Requests,
+but it may make sense to manually perform tests (possibly with our `test_pr` script),
+particularly for PRs that affect `IPython.parallel` or Windows.
+
+For more detailed information, see our [GitHub Workflow](https://github.com/ipython/ipython/wiki/Dev:-GitHub-workflow).
+