add GitHub templates

Author: Christopher Willis-Ford

.github/CONTRIBUTING.md

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+## Contributing
+
+The development of Scratch is an ongoing process, and we love to have people in the Scratch and open source
+communities help us along the way.
+
+If you're interested in contributing, please take a look at the
+[issues](https://github.com/LLK/scratch-desktop/issues) on this repository.
+
+Two great ways of helping are by identifying bugs and documenting them as issues, or fixing issues and creating pull
+requests. When looking for bugs to fix, please look for the ["Help Wanted"
+label](https://github.com/LLK/scratch-desktop/issues?q=label%3A%22help+wanted%22). Bugs with this label have been
+specifically set aside for Open Source contributors. Issues without the label can also be worked on but we ask that
+you comment on the issue prior to starting work. When submitting pull requests please be patient -- it can take a
+while to find time to review them. The organization and class structures can't be radically changed without
+significant coordination and collaboration from the Scratch Team, so these types of changes should be avoided.
+
+It's been said that the Scratch Team spends about one hour of design discussion for every pixel in Scratch, but some
+think that estimate is a little low. While we welcome suggestions for new features in our [suggestions
+forum](https://scratch.mit.edu/discuss/1/) (especially ones that come with mockups), we are unlikely to accept PRs
+with new features that haven't been thought through and discussed as a group. Why? Because we have a strong belief in
+the value of keeping things simple for new users. To learn more about our design philosophy, see [the Scratch
+Developers page](https://scratch.mit.edu/developers), or [this
+paper](http://web.media.mit.edu/~mres/papers/Scratch-CACM-final.pdf).
+
+Beyond this repo, there are also some other resources that you might want to take a look at:
+
+* [Community Guidelines](https://github.com/LLK/scratch-www/wiki/Community-Guidelines) (we find it important to
+  maintain a constructive and welcoming community, just like on Scratch)
+* [Open Source forum](https://scratch.mit.edu/discuss/49/) on Scratch
+* [Suggestions forum](https://scratch.mit.edu/discuss/1/) on Scratch
+* [Bugs & Glitches forum](https://scratch.mit.edu/discuss/3/) on Scratch

.github/ISSUE_TEMPLATE.md

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+### Expected Behavior
+
+_Please describe what should happen_
+
+### Actual Behavior
+
+_Describe what actually happens_
+
+### Steps to Reproduce
+
+_Explain what someone needs to do in order to see what's described in *Actual behavior* above_
+
+### Operating System and Browser
+
+_e.g. Mac OS 10.11.6 Safari 10.0_

.github/PULL_REQUEST_TEMPLATE.md

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+### Resolves
+
+_What Github issue does this resolve (please include link)?_
+
+- Resolves #
+
+### Proposed Changes
+
+_Describe what this Pull Request does_
+
+### Reason for Changes
+
+_Explain why these changes should be made_
+
+### Test Coverage
+
+_Please show how you have added tests to cover your changes_