pull in CONTRIBUTING.md from the wiki

this will cause it to show up as a link from incoming pull requests, and
also make the documentation and its change history a little more
first-class with respect to the rest of the project.

big ups to @mkouhia, @passcod, @mwarkentin, and @vincecima for their
work on this document!

CONTRIBUTING.md

@@ -0,0 +1,151 @@
+# How to Contribute
+
+So you want to contribute to the project. THIS IS GREAT NEWS!
+
+Seriously. We're all pretty happy about this.
+
+## Getting set up to contribute
+
+Fork the homebrew-cask repository as per instructions in the
+[GitHub help pages](https://help.github.com/articles/fork-a-repo):
+
+1. Fork the repository in GitHub with the 'Fork' button
+2. Clone your GitHub fork into your computer
+3. Configure upstream remote to point at
+  [phinze/homebrew-cask](https://github.com/phinze/homebrew-cask.git).
+
+## Adding a Cask
+
+Making a Cask is easy: a Cask is a Ruby file that is only seven lines long.
+See the Cask for Alfred.app for an example:
+
+  ```ruby
+  class Alfred < Cask
+    url 'http://cachefly.alfredapp.com/alfred_1.3.2_265.zip'
+    homepage 'http://www.alfredapp.com/'
+    version '1.3.2_265'
+    sha1 'e10f15a0ae758cae2a22557f14890fa5989260ce'
+  end
+  ```
+
+### Preliminary steps
+
+1. Go to the folder in which you cloned `phinze/homebrew-cask` (assuming
+  `~/homebrew-cask`)
+  ```bash
+  $ cd ~/homebrew-cask
+  ```
+2. If you have forked a longer while ago, there might be upstream changes.
+  Pull those in (assuming upstream branch is called `upstream`):
+  ```bash
+  $ git fetch upstream
+  # Fetches any new changes from the original repo
+  $ git checkout master
+  # Check out branch "master" (just to make sure)
+  $ git merge upstream/master
+  # Merges any changes fetched into your working files
+  ```
+3. Make a topic branch for your work
+  ```bash
+  $ git checkout -b my-branch
+  # Create a new branch called "my-branch" and make it active
+  ```
+
+### Write the Cask file
+
+1. Copy a template cask to help writing the new one
+  ```bash
+  $ cp test/support/Casks/basic-cask.rb Casks/my-app.rb
+  ```
+2. Edit the Cask with your preferred editor
+  ```bash
+  $ emacs Casks/my-app.rb
+  ```
+  Include the following fields in your Cask:
+    * URL to the `.dmg`/`.zip` file that contains the application (currently
+      only `.app` files are supported)
+    * Application homepage
+    * Application version
+    * Checksum of the file
+
+  In order to find out the checksum for the file, you need to download the file
+  and then:
+  
+  ``` bash
+  $ curl -OL http://example-server.com/path/to/file/my-app.dmg
+  # Download the file that contains the application
+  $ shasum my-app.dmg
+  # Calculate SHA-1 sum for the downloaded file
+  ```
+  
+  Some developers (like Mozilla) and hosts (like Sourceforge) provide checksums,
+  so you can use those directly, without needing to download the file separately.
+  
+  In addition to `sha1` you can provide the checksum with `md5`, `sha256`, or `sha512`
+  in the same manner.  Use commands `md5 my-app.dmg` or `shasum -a 256 my-app.dmg`
+  to obtain these checksums.
+
+  If the application does not have versioned downloads, you can provide option
+  `no_checksum`, which takes no arguments.
+
+This is all you need to do to write a Cask!
+
+## Testing your new Cask
+
+When Casks are installed, they are searched in `/usr/local/Library/Taps`.  You
+can replace the files in folder `phinze-cask` but you are recommended to make
+your own folder in which you place the tested Casks.
+  
+  ```bash
+  $ mkdir -p /usr/local/Library/Taps/my-casks/Casks
+  # Create directory in which the tested Casks are placed
+  $ ln -s ~/homebrew-cask/Casks/my-app.rb /usr/local/Library/Taps/my-casks/Casks
+  # Create a symbolic link to the Cask you created
+  ```
+
+Now you can either audit your Cask with `brew cask audit my-app` or try to
+install it: `brew cask install my-app`.
+
+Did it install?  If yes, you can continue with submitting a pull request.  If
+something went wrong, `brew cask uninstall my-app` and edit your Cask in
+`~/homebrew-cask/Casks/my-app.rb` or whereever you placed it.  Since you
+created a symbolic link in `/usr/local/Library/Taps/my-casks/Casks`, changes
+are automatically reflected in there, so you can directly try installing after
+those changes.
+
+If your application and homebrew-cask do not work well together, feel free to
+[file an issue](https://github.com/phinze/homebrew-cask/issues) after checking
+out open issues.
+
+## Submitting your Cask
+
+Now that you have your Cask ready and you have verified it works correctly, you
+can commit your changes and submit a
+[pull request](https://help.github.com/articles/using-pull-requests).
+
+### Committing your changes
+
+Go back to the folder in which you cloned homebrew-cask.  Use `git status` to
+verify that you are on the right branch and that you have changed the right
+files.
+
+1. Stage your Cask with `git add Casks/my-app.rb`.  You can view the changes
+  that are to be committed with `git diff --cached`.
+2. Commit your changes with `git commit`.  Write your commit message with
+    * the first line being commit summary, 50 characters or less,
+    * followed by an empty line
+    * and an explanation of the commit, wrapped to 72 characters.
+  See
+  [a note about git commit messages](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html)
+  for a more thorough explanation.
+3. Push your changes to your GitHub account: `git push origin my-branch`.
+
+### Filing a pull request on GitHub
+
+Now go to *your* GitHub repository at
+https://github.com/your-github-username/homebrew-cask, switch branch to your
+topic branch and click on 'Pull Request' button.  You can then add further
+comments to your pull request.
+
+You are done now, and your Cask should be pulled in or otherwise noticed in a
+while.