add a CONTRIBUTING.md. closes mochajs#882

CONTRIBUTING.md

@@ -0,0 +1,49 @@
+# Contributing to Mocha
+
+Hi!  We could use your help.  Let us help you help us.  Or something.
+
+## General
+
+1. If you are looking for a place to begin, **please send PRs for bugfixes instead of new features**, and/or **look for issues labeled `PR PLEASE`.**
+
+2.  **Help with documentation and the wiki is always appreciated**.
+
+3.  Please **be courteous and constructive** when commenting on issues, commits, and pull requests.
+
+## Bug Reports & Issues
+
+1.  When reporting a bug, please **provide steps to reproduce**.  If possible, show code.
+
+2.  Please **show all code in JavaScript**.  We don't all read <insert-language-that-compiles-to-JavaScript-here>.  If you do not, you will be asked to.
+
+3.  Because Mocha works with many third-party libraries and tools, **ensure the bug you are reporting is actually within Mocha**.
+
+4.  If you report a bug, and it is inactive for a significant amount of time, it may be closed.  **Please respond promptly to requests for more information**.
+
+## Pull Requests
+
+1. Before sending a large PR, it's recommended to **create an issue to propose the change**.  Nobody wants to write a book of code and throw it away.
+
+2.  Because Mocha should be kept as maintainable as possible, its codebase must be kept slim.  Historically, *most PRs for new features are not merged*.  New features inevitably increase the size of the codebase, and thus reduce maintainability.  Only features *deemed essential* are likely to be merged--this is at the discretion of the maintainer(s).  If your PR for a feature is not merged, this doesn't necessarily mean your PR was a bad idea, wouldn't be used, or otherwise sucks.  It just means **only essential PRs for new features are likely to be merged**. 
+
+3.  Due to the above, before creating a PR for a new feature, **create an issue to propose the feature.**
+
+4.  Please **respect existing coding conventions**, whatever those may be.
+
+5.  If your PR has been waiting in limbo for some time, it's very helpful to **rebase against master**, which will make it easier to merge.
+
+6.  Please **add tests for new code**.
+
+7.  **Always run `npm test` before sending a PR.  If you break the tests, your PR will not be accepted until they are fixed.
+
+## Source Control
+
+1. Please **squash your commits** when sending a pull request.  If you are unfamiliar with this process, see [this guide](https://help.github.com/articles/about-git-rebase/).  If you have already pushed your changesets and are squashing thereafter, this may necessitate the use of a "force push".  Please [read the docs](http://git-scm.com/docs/git-push) before you attempt this. 
+
+2. Please **follow the commit message conventions [outlined here](https://medium.com/code-adventures/git-conventions-a940ee20862d).**
+
+## TL;DR
+
+**Be kind, be diligent, look before you leap into a PR, and follow common community conventions**.
+
+*- The Mocha Team*