add contributing.md

contributing.md

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+# Contributing
+
+When making a pull request for this repo, please make sure of a few things
+
+- tests and linting should pass for you locally. We have CI tests that also enforce this.
+- rebuild the shrinkwrap file if you're changing any dependencies.
+
+## Rebuilding the shrinkwrap
+
+Because of the differences beween npm versions 2 and 3, you will want to use npm 2.  A shrinkwrap built under npm 2 will also work under npm 3.  A shrinkwrap built under npm 3 will *not* work under npm 2.
+
+The simplest way to build a new shrinkwrap is to start with an empty node_modules.  Once you've done that and have made sure you're using npm 2:
+
+```sh
+$ npm install
+$ npm run shrinkwrap
+```
+
+Note that it is `npm run shrinkwrap` not `npm shrinkwrap`.  This is because we have a shrinkwrap script that not only runs the shrinkwrap itself but also runs `shrinkydink`, a post-processor that cleans out some unneeded info we don't want.