Add PR template

.github/CONTRIBUTING.md

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+# Before You Submit an Issue
+
+- Are you using Windows? We unfortunately don't support Windows. You could try using Docker, as outlined in the `README`.
+- Is your version of Slate out of date? We have [upgrade instructions](https://github.com/tripit/slate/wiki/Updating-Slate) in the wiki. You could also try seeing if your problem is reproducible on the latest version of Slate.
+
+If you answered "no" to each of the questions above, feel free to submit an issue! It's also helpful if you include a code example of your problem (if applicable), we can't help you if you just say "Slate stopped loading for me once I added my documentation" without telling us the problematic documentation.
+
+# Before You Submit a Pull Request
+
+Thanks for contributing to Slate! A couple of quick guidelines for submitting pull requests:
+
+- **Please point your pull requests at the `dev` branch.** We don't accept pull requests to `master`.
+- Please make sure your contributions work in the most recent version of Chrome, Firefox, and IE.
+- If you're implementing a new feature, even if it's relatively small, it's nice to open an issue before you start so that others know what you're working on and can help make sure you're on the right track.
+
+Thanks again! Happy coding.

.github/PULL_REQUEST_TEMPLATE.md

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+Make sure you've checked off all these things before submitting:
+
+- [ ] This pull request isn't for a company's fork, it's intended for the upstream Slate shared by everybody.
+- [ ] This pull request is submitted to the `dev` branch.
+- [ ] If it makes frontend changes, this pull request has been tested in the latest version of Firefox, Chrome, IE, and Safari.