Add contribution guidelines

CONTRIBUTING.md

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+# Contributing to Mongojs
+
+  :+1::tada: First off, thanks for taking the time to contribute! :tada::+1:
+
+The following is a set of guidelines for contributing to mongojs.
+These are just guidelines, not rules, use your best judgment and feel free to
+propose changes to this document in a pull request.
+
+  This project adheres to the [Open Code of Conduct][code-of-conduct]. By participating, you are expected to uphold this code.
+  [code-of-conduct]: http://todogroup.org/opencodeofconduct/#mongojs/mathiasbuus@gmail.com
+
+## Submitting Issues
+
+* You can create an issue [here](https://github.com/mafintosh/mongojs/issues/new), but
+before doing that please read the notes below on debugging and submitting issues,
+  and include as many details as possible with your report.
+* Include the version of mongojs you are using.
+* Include the version of mongodb you are using and the OS.
+* Include some source code snippet to demo the issue
+* Include a stack trace and error message if applicable
+
+## Pull Requests
+
+* Follow the [Standard](https://github.com/feross/standard) style.
+* Include tests in the `test` directory.
+* Update `README.md` if your pull request adds/changes functionality
+
+## Git Commit Messages
+
+* Use the present tense ("Add feature" not "Added feature")
+* Use the imperative mood ("Open database when..." not "Opens database when...")
+* Limit the first line to 72 characters or less
+* Reference issues and pull requests liberally
+* Consider starting the commit message with an applicable emoji:
+    * :art: `:art:` when improving the format/structure of the code
+    * :racehorse: `:racehorse:` when improving performance
+    * :non-potable_water: `:non-potable_water:` when plugging memory leaks
+    * :memo: `:memo:` when writing docs
+    * :bug: `:bug:` when fixing a bug
+    * :fire: `:fire:` when removing code or files
+    * :green_heart: `:green_heart:` when fixing the CI build
+    * :white_check_mark: `:white_check_mark:` when adding tests
+    * :lock: `:lock:` when dealing with security
+    * :arrow_up: `:arrow_up:` when upgrading dependencies
+    * :arrow_down: `:arrow_down:` when downgrading dependencies
+    * :shirt: `:shirt:` when removing linter warnings