feat(.): add first pass at contributing & issue & pr templates

CONTRIBUTING.md

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+# Contributing to Lean
+
+### Writing Code
+
+- [Coding Style](doc/coding_style.md)
+- [Library Style Conventions](doc/lean/library_style.org)
+- [Git Commit Conventions](doc/commit_convention.md)
+- [Automatic Builds](doc/make/travis.md)
+
+# Opening Issues
+
+-  Checked that your issue isn't [already filed](https://github.com/leanprover/lean/issues).
+    * Look over the wishlist the [wishlist](https://github.com/leanprover/lean/issues?q=is%3Aissue+is%3Aopen+label%3AI-wishlist), open [RFCs](https://github.com/leanprover/lean/issues?q=is%3Aissue+is%3Aopen+label%3ARFC),
+    or [feature requests](https://github.com/leanprover/lean/issues?q=is%3Aissue+is%3Aopen+label%3AFeature).
+    * Checked that there is not already an Atom package that provides the described functionality: https://atom.io/packages
+    * Reduced the issue to a self-contained, reproducible test case.
+
+# Opening Pull Requests
+
+- Ensure your Pull Request meets the coding and commit conventions listed above.
+- Ensure your Pull Request contains tests for the behavior whether a feature or
+  bug fix.
+
+

ISSUE_TEMPLATE.md

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+### Prerequisites
+
+* [ ] Put an X between the brackets on this line if you have done all of the following:
+    * Checked that your issue isn't already [filed](https://github.com/leanprover/lean/issues).
+        * Specifially, check out the [wishlist](https://github.com/leanprover/lean/issues?q=is%3Aissue+is%3Aopen+label%3AI-wishlist), open [RFCs](https://github.com/leanprover/lean/issues?q=is%3Aissue+is%3Aopen+label%3ARFC),
+        or [feature requests](https://github.com/leanprover/lean/issues?q=is%3Aissue+is%3Aopen+label%3AFeature).
+    * Reduced the issue to a self-contained, reproducible test case.
+
+### Description
+
+[Description of the issue]
+
+### Steps to Reproduce
+
+1. [First Step]
+2. [Second Step]
+3. [and so on...]
+
+**Expected behavior:** [What you expect to happen]
+
+**Actual behavior:** [What actually happens]
+
+**Reproduces how often:** [What percentage of the time does it reproduce?]
+
+### Versions
+
+You can get this information from copy and pasting the output of `lean --version`,
+please include the OS and what version of the OS you're running.
+
+### Additional Information
+
+Any additional information, configuration or data that might be necessary to reproduce the issue.

PULL_REQUEST_TEMPLATE.md

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+# Pull Request Description
+
+Ensure you have read the contribution guide before filling in a description of the
+pull request, regardless of whether it is complete or a work in progress.
+All Pull Requests should includ test case(s) which demonstrates the intended
+behavior of a feature, or a regression test demonstrating that the fix resolves
+the issue.