add CONTRIBUTING.md and modify README.md

CONTRIBUTING.md

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+# How to contribute #
+
+We'd love to accept your patches and contributions to this project.  There are
+a just a few small guidelines you need to follow.
+
+## Submitting a patch ##
+
+  1. It's generally best to start by opening a new issue describing the bug or
+     feature you're intending to fix.  Even if you think it's relatively minor,
+     it's helpful to know what people are working on.  Mention in the initial
+     issue that you are planning to work on that bug or feature so that it can
+     be assigned to you.
+
+  1. Follow the normal process of [forking][] the project, and setup a new
+     branch to work in.  It's important that each group of changes be done in
+     separate branches in order to ensure that a pull request only includes the
+     commits related to that bug or feature.
+
+  1. Do your best to have [well-formed commit messages][] for each change.
+     This provides consistency throughout the project, and ensures that commit
+     messages are able to be formatted properly by various git tools.
+
+  1. Finally, push the commits to your fork and submit a [pull request][].
+
+[forking]: https://help.github.com/articles/fork-a-repo
+[well-formed commit messages]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
+[pull request]: https://help.github.com/articles/creating-a-pull-request
+
+## Coding Style ##
+
+* Go code must be run through 'go fmt'.
+* Follow http://golang.org/doc/effective_go.html as much as possible.
+    + In particular, http://golang.org/doc/effective_go.html#mixed-caps.  Enums
+      should be be CamelCase, with acronyms capitalized (TCPSourcePort, vs.
+      TcpSourcePort or TCP_SOURCE_PORT).
+* Bonus points for giving enum types a String() field.
+* Any exported types or functions should have commentary
+   (http://golang.org/doc/effective_go.html#commentary)