Pull request template (#4994)

Author: GeoffreyBooth

PULL_REQUEST_TEMPLATE.md

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+<!--
+Before making a PR please make sure to read our contributing guidelines:
+http://coffeescript.org/#contributing
+
+For issue references: Add a comma-separated list of a 
+[closing word](https://help.github.com/articles/closing-issues-via-commit-messages/) followed by 
+the ticket number fixed by the PR. It should be underlined in the preview if done correctly.
+
+All new features require tests. All but the most trivial bug fixes should also have new or updated tests.
+
+Ensure that all new code you add to the compiler can be run in the minimum version of Node listed in 
+`package.json`. New tests can require newer Node runtimes, but you may need to ensure that such tests
+only run in supported runtimes; see `Cakefile` for examples of how to filter out certain tests in 
+runtimes that don’t support them.
+
+Please follow the code style of the rest of the CoffeeScript codebase. Write comments in complete
+sentences using Markdown, as the comments become the [annotated source](http://coffeescript.org/#annotated-source).
+For tests proving a bug is fixed, please mention the issue number in the test description (see examples
+in the codebase).
+
+Describe your changes below in as much detail as possible.
+-->