Add CONTRIBUTING.md and GitHub templates

.github/ISSUE_TEMPLATE.md

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+### Problem Description
+
+Describe the problem you are having, what you expect to happen
+instead, and how to reproduce the problem.
+
+### Configuration File
+
+Please include the smallest configuration file that reproduces the
+problem you are experiencing:
+
+```haskell
+module Main (main) where
+
+import XMonad
+
+main :: IO ()
+main = xmonad def
+```
+
+### Checklist
+
+  - [ ] I've read [CONTRIBUTING.md](https://github.com/xmonad/xmonad/blob/master/CONTRIBUTING.md)
+
+  - [ ] I tested my configuration with [xmonad-testing](https://github.com/xmonad/xmonad-testing)

.github/PULL_REQUEST_TEMPLATE.md

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+### Description
+
+Include a description for your changes, including the motivation
+behind them.
+
+### Checklist
+
+  - [ ] I've read [CONTRIBUTING.md](https://github.com/xmonad/xmonad/blob/master/CONTRIBUTING.md)
+
+  - [ ] I've confirmed these changes don't belong in xmonad-contrib instead
+
+  - [ ] I tested my changes with [xmonad-testing](https://github.com/xmonad/xmonad-testing)
+
+  - [ ] I updated the `CHANGES.md` file

CONTRIBUTING.md

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+# Contributing to xmonad and xmonad-contrib
+
+## Before Creating a GitHub Issue
+
+New issue submissions should adhere to the following guidelines:
+
+  * Does your issue have to do with [xmonad][], [xmonad-contrib][], or
+    maybe even with the [X11][] library?
+
+    Please submit your issue to the **correct** GitHub repository.
+
+  * To help you figure out which repository to submit your issue to,
+    and to help us resolve the problem you are having, create the
+    smallest configuration file you can that reproduces the problem.
+
+    You may find that the [xmonad-testing][] repository is helpful in
+    reproducing the problem with a smaller configuration file.
+
+    Once you've done that please include the configuration file with
+    your GitHub issue.
+
+  * If possible, use the [xmonad-testing][] repository to test your
+    configuration with the bleeding-edge development version of xmonad
+    and xmonad-contrib.  We might have already fixed your problem.
+
+## Contributing Changes/Patches
+
+Have a change to xmonad that you want included in the next release?
+Awesome!  Here are a few things to keep in mind:
+
+  * Review the above section about creating GitHub issues.
+
+  * It's always best to talk with the community before making any
+    nontrivial changes to xmonad.  There are a couple of ways you can
+    chat with us:
+
+    - Post a message to the [mailing list][ml].
+
+    - Join the `#xmonad` IRC channel on `chat.freenode.org`.
+
+  * Continue reading this document!
+
+## Expediting Reviews and Merges
+
+Here are some tips for getting your changes merged into xmonad:
+
+  * If your changes can go into [xmonad-contrib][] instead
+    of [xmonad][], please do so.  We rarely accept new features to
+    xmonad.  (Not that we don't accept changes to xmonad, just that we
+    prefer changes to xmonad-contrib instead.)
+
+  * Change the fewest files as possible.  If it makes sense, submit a
+    completely new module to xmonad-contrib.
+
+  * Your changes should include relevant entries in the `CHANGES.md`
+    file.  Help us communicate changes to the community.
+
+  * Make sure you test your changes using the [xmonad-testing][]
+    repository.  Include a new configuration file that shows off your
+    changes if possible by creating a PR on that repository as well.
+
+[xmonad]: https://github.com/xmonad/xmonad
+[xmonad-contrib]: https://github.com/xmonad/xmonad-contrib
+[xmonad-testing]: https://github.com/xmonad/xmonad-testing
+[x11]: https://github.com/xmonad/X11
+[ml]: https://mail.haskell.org/cgi-bin/mailman/listinfo/xmonad