Add contributing guidelines

See https://github.com/blog/1184-contributing-guidelines

CONTRIBUTING.md

+# How to contribute
+
+- [Getting help](#getting-help-)
+- [Submitting bug reports](#submitting-bug-reports-)
+- [Contributing code](#contributing-code-)
+
+## Getting help [^](#how-to-contribute)
+
+Community discussion, questions, and informal bug reporting is done on the
+[CodeMirror Google group](http://groups.google.com/group/codemirror).
+
+## Submitting bug reports [^](#how-to-contribute)
+
+The preferred way to report bugs is to use the
+[GitHub issue tracker](http://github.com/marijnh/CodeMirror/issues). Before
+reporting a bug, read these pointers.
+
+**Note:** The issue tracker is for *bugs*, not requests for help. Questions
+should be asked on the
+[CodeMirror Google group](http://groups.google.com/group/codemirror) instead.
+
+### Reporting bugs effectively
+
+- CodeMirror is maintained by volunteers. They don't owe you anything, so be
+  polite. Reports with an indignant or belligerent tone tend to be moved to the
+  bottom of the pile.
+
+- Include information about **the browser in which the problem occurred**. Even
+  if you tested several browsers, and the problem occurred in all of them,
+  mention this fact in the bug report. Also include browser version numbers and
+  the operating system that you're on.
+
+- Mention which release of CodeMirror you're using. Preferably, try also with
+  the current development snapshot, to ensure the problem has not already been
+  fixed.
+
+- Mention very precisely what went wrong. "X is broken" is not a good bug
+  report. What did you expect to happen? What happened instead? Describe the
+  exact steps a maintainer has to take to make the problem occur. We can not
+  fix something that we can not observe.
+
+- If the problem can not be reproduced in any of the demos included in the
+  CodeMirror distribution, please provide an HTML document that demonstrates
+  the problem. The best way to do this is to go to
+  [jsbin.com](http://jsbin.com/ihunin/edit), enter it there, press save, and
+  include the resulting link in your bug report.
+
+## Contributing code [^](#how-to-contribute)
+
+- Make sure you have a [GitHub Account](https://github.com/signup/free)
+- Fork [CodeMirror](https://github.com/marijnh/CodeMirror/)
+  ([how to fork a repo](https://help.github.com/articles/fork-a-repo))
+- Make your changes
+    - If your change affects highlighting for one of the modes, please [add (or
+    change) tests](#adding-mode-highlighting-tests) for the changes. If the mode
+    doesn't already have highlighting tests, you *aren't* required to add any.
+- Test your changes
+    -Visit `/path-to-code/test/index.html` to test your code. *All tests should
+    pass*.
+- Submit a pull request
+([how to create a pull request](https://help.github.com/articles/fork-a-repo))
+
+### Adding mode highlighting tests
+
+- Create a `test.js` file in the corresponding mode directory
+   ([example](https://github.com/marijnh/CodeMirror/blob/master/mode/markdown/test.js))
+- Add script tags to `/test/index.html` to include the formatting code and
+   as well as the tests.
+- Run the tests!
+
+### Code formatting standards
+
+- 2 spaces (no tabs)
+- Wrap to 80 characters when possible (unless it affects readability negatively)
+- No trailing whitespace
+    - Blank lines should be indented as if there *is* text on them
+- Spacing
+    - `function someFunction(someVar, someOtherVar) {`
+    - `if (someVar === true) doThis(someVar, someOtherVar);`
+    - `if (!someVar || someOtherVar === 0) {`

README.md

@@ -4,5 +4,6 @@ CodeMirror is a JavaScript component that provides a code editor in
 the browser. When a mode is available for the language you are coding
 in, it will color your code, and optionally help with indentation.
 
-The project page is http://codemirror.net
-The manual is at http://codemirror.net/doc/manual.html
+The project page is http://codemirror.net  
+The manual is at http://codemirror.net/doc/manual.html  
+The contributing guidelines are in the CONTRIBUTING.md file