Added a contributing guide.

CONTRIBUTING.md

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+# Contributing
+
+Everybody is more than welcome to contribute to Oga, no matter how small the
+change. To keep everything running smoothly there are a few guidelines that one
+should follow. These are as following:
+
+* When changing code make sure to write RSpec tests for the changes.
+* Document code using YARD. At the very least the method arguments and return
+  value(s) should be documented.
+* Wrap lines at 79 characters per line.
+* Git commits should have a <= 50 character summary, optionally followed by a
+  blank line and a more in depth description of 79 characters per line.
+
+## Editor Setup
+
+Whatever editor you use doesn't matter as long as it can do two things:
+
+* Use spaces for indentation.
+* Hard wrap lines at 79 characters per line.
+
+To make this process easier Oga comes with an [EditorConfig][editorconfig]
+configuration file. If your editor supports this it will automatically apply
+the required settings for you.
+
+## Hacking on Oga
+
+Assuming you have a local Git clone of Oga, the first step should be to install
+all the required Gems:
+
+    bundle install
+
+Next up, compile the required files and run the tests:
+
+    rake
+
+You can compile the various parsers/extensions by running:
+
+    rake generate
+
+For more information about the available tasks, run `rake -T`.
+
+## Extension Setup
+
+Oga uses native extensions for the XML lexer. This is due to Ruby sadly not
+being fast enough to chew through large amounts of XML (at least when using
+Ragel). For example, the benchmark `benchmark/lexer/big_xml_time.rb` would take
+around 6 seconds to complete on MRI 2.1.1. The native extensions on the other
+hand can complete this benchmark in roughly 600 milliseconds.
+
+Oga has two native extensions: one for MRI/Rubinius (written in C) and one for
+JRuby (written in Java). Both extensions share the same Ragel grammar, found in
+`ext/ragel/base_lexer.rl`. This grammar is set up in such a way that the syntax
+is compatible with both C and Java. Specific details on how the grammar is used
+can be found in the documentation of said grammar file.
+
+The native extensions call back in to Ruby to actually perform the task of
+creating tokens, validating input and so forth. As a result of this you'll most
+likely never have to touch the C and/or Java code when changing the behaviour
+of the lexer.
+
+To compile the extensions run `rake generate` using your Ruby implementation of
+choice. Note that extensions compiled for MRI can not be used on Rubinius and
+vice-versa. To compile the JRuby extension you'll have to switch your active
+Ruby version to JRuby first.
+
+## Contact
+
+In case you have any further questions or would like to receive feedback before
+submitting a change, feel free to contact me. You can either open an issue,
+send a tweet to [@yorickpeterse][twitter] or send an Email to
+<yorickpeterse@gmail.com>.
+
+## Legal
+
+By contributing to Oga you agree with both the Developer's Certificate of
+origin, found in `doc/DCO.md` and the license, found in `LICENSE`. This applies
+to all content in this repository unless stated otherwise.
+
+[editorconfig]:http://editorconfig.org/
+[twitter]: https://twitter.com/yorickpeterse