add issue and pull request templates

.github/ISSUE_TEMPLATE.md

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+<!--
+Thanks for raising an issue! To help us help you, if you've found a bug please consider the following:
+
+* If you can demonstrate the bug using the interactive [rollupjs.org](http://rollupjs.org) demo, please do.
+* If that's not possible, perhaps because your bug involves plugins, we recommend creating a small repo that illustrates the problem.
+
+Reproductions should be [small, self-contained, correct examples](http://sscce.org/). For example, if your bug isn't related to Gulp, **do not include Gulp in the repro**. (Side-note: if it *is* related to Gulp, this is the wrong issue tracker!) If possible, use a [config file](https://github.com/rollup/rollup/wiki/Command-Line-Interface#using-a-config-file) as it's the standard, favoured approach to using Rollup.
+
+Occasionally, this won't be possible, and that's fine – we still appreciate you raising the issue. But please understand that Rollup is run by unpaid volunteers in their free time, and we will prioritise bugs that follow these instructions.
+
+If you have a stack trace to include, we recommend putting inside a `<details>` block for the sake of the thread's readability:
+
+<details>
+  <summary>Stack trace</summary>
+
+  Stack trace goes here...
+</details>
+-->

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+Thank you for creating a pull request. Before submitting, please note the following:
+
+* If your pull request implements a new feature, please raise an issue to discuss it before sending code. In many cases features are absent for a reason.
+* This message body should clearly illustrate what problems it solves. If there are related issues, remember to reference them.
+* If necessary, and you know how, consider squashing your commits before merging. (This isn't essential but it keeps the history cleaner.)
+* Ideally, include a test that fails without this PR but passes with it. PRs will only be merged once they pass CI. (Remember to `npm run lint`!)
+-->