Add docs for contributors. (ray-project#1191)

* WIP: add docs for contributors.

* Remove changelog part.

* Simplify issue template.

* Simplify pull request template

* Simplify contributing doc.

.github/ISSUE_TEMPLATE.md

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+<!--
+General questions should be asked on the mailing list [email protected].
+
+Before submitting an issue, please fill out the following form.
+-->
+
+### System information
+- **OS Platform and Distribution (e.g., Linux Ubuntu 16.04)**:
+- **Ray installed from (source or binary)**:
+- **Ray version**:
+- **Python version**:
+- **Exact command to reproduce**:
+
+<!--
+You can obtain the Ray version with
+
+python -c "import ray; print(ray.__version__)"
+-->
+
+### Describe the problem
+<!-- Describe the problem clearly here. -->
+
+### Source code / logs
+<!-- Include any logs or source code that would be helpful to diagnose the problem. If including tracebacks, please include the full traceback. Large logs and files should be attached. Try to provide a reproducible test case that is the bare minimum necessary to generate the problem. -->

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+Thank you for your contribution!
+
+Please review https://github.com/ray-project/ray/blob/master/CONTRIBUTING.rst before opening a pull request.
+-->
+
+## What do these changes do?
+
+<!-- Please give a short brief about these changes. -->
+
+## Related issue number
+
+<!-- Are there any issues opened that will be resolved by merging this change? -->

CONTRIBUTING.rst

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+Contributing to Ray
+===================
+
+Reporting bugs and asking questions
+-----------------------------------
+
+We use `GitHub issues`_ for bug reports and feature requests. For discussions
+about development, questions about usage, or any general questions, we use our
+mailing list `[email protected]`_.
+
+To contribute a patch:
+----------------------
+
+1. Break your work into small, single-purpose patches if possible. It’s much
+   harder to merge in a large change with a lot of disjoint features.
+2. Submit the patch as a GitHub pull request against the master branch.
+3. Make sure that your code passes the unit tests.
+4. Add new unit tests for your code.
+
+.. _`GitHub issues`: https://github.com/ray-project/ray/issues
+.. _`[email protected]`: https://groups.google.com/forum/#!forum/ray-dev