Add a .github directory with contribution guidelines and templates

.github/CONTRIBUTING.md

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+# Contribution Guidelines
+
+PEG.js is an open source project and it welcomes contributions. To make sure
+your contributions are useful and the overall process is smooth, please adhere
+to these guidelines.
+
+You can contribute in three main ways: by reporting bugs, requesting features,
+and contributing code.
+
+## Reporting Bugs
+
+You can report bugs using [GitHub issues][issues]. Before submitting a bug
+report, [search the existing reports][issues-search-bugs] to see if the bug
+wasn’t reported already.
+
+Each bug report should contain this information:
+
+  * Steps to reproduce
+  * What you expected to happened
+  * What happened instead
+
+In most cases, it is useful to include a **minimal** example (grammar + input)
+reproducing the problem.
+
+## Requesting Features
+
+You can request features using [GitHub issues][issues]. Before submitting a
+feature request, [search the existing requests][issues-search-enhancements] to
+see if the feature wasn’t requested already.
+
+Each feature request should contain this information:
+
+  * Feature description
+  * Why do you request it
+
+## Contributing Code
+
+You can contribute code using [GitHub pull requests][pulls]. However, for
+anything but trivial changes, file an issue first. Your idea may not fit the
+project, or the implementation you have in mind may not be the right one. Filing
+an issue in these cases will prevent wasting time and effort on both sides.
+
+Split your change into atomic commits with descriptive commit messages adhering
+to [these conventions][git-commit-messages]. Have a look in the commit history
+to see good examples.
+
+When appropriate, add documentation and tests.
+
+Before submitting, make sure your change passes the specs (`make spec`) and
+ESLint checks (`make lint`).
+
+[issues]: https://github.com/pegjs/pegjs/issues
+[issues-search-bugs]: https://github.com/pegjs/pegjs/issues?q=is%3Aopen+is%3Aissue+label%3ABug
+[issues-search-enhancements]: https://github.com/pegjs/pegjs/issues?q=is%3Aopen+is%3Aissue+label%3AEnhancement
+[pulls]: https://github.com/pegjs/pegjs/pulls
+[git-commit-messages]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html

.github/ISSUE_TEMPLATE.md

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+Checklist:
+
+  * [ ] This is not a question (these belong to the [Google Group][google-group])
+  * [ ] I searched existing issues to make sure I don’t create a duplicate one
+
+For bug reports, please include:
+
+  * Steps to reproduce
+  * What I expected to happened
+  * What happened instead
+
+In most cases, it is also useful to include a **minimal** example (grammar + input)
+reproducing the problem.
+
+For feature requests, please include:
+
+  * Feature description
+  * Why do you request it
+
+[google-group]: https://groups.google.com/forum/#!forum/pegjs

.github/PULL_REQUEST_TEMPLATE.md

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+Checklist:
+
+  * [ ] The change is trivial (if not, I filed an issue first)
+  * [ ] The change is split into atomic commits
+  * [ ] Commits have messages adhering to [these conventions][git-commit-messages]
+  * [ ] The change is documented (if needed)
+  * [ ] The change is tested (if needed)
+  * [ ] The change passes the specs (`make spec`)
+  * [ ] The change passes ESLint checks (`make lint`)
+
+[git-commit-messages]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html