Merge pull request #305 from jfsiii/fix-correct-contributing-md

Don't assume `/usr/local`. Use `brew --prefix`.

CONTRIBUTING.md

@@ -93,14 +93,14 @@ 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
+When Casks are installed, they are searched in `` `brew --prefix`/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
+  $ mkdir -p `brew --prefix`/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
+  $ ln -s ~/homebrew-cask/Casks/my-app.rb `brew --prefix`/Library/Taps/my-casks/Casks
   # Create a symbolic link to the Cask you created
   ```
 
@@ -110,7 +110,7 @@ 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
+created a symbolic link in `` `brew --prefix`/Library/Taps/my-casks/Casks ``, changes
 are automatically reflected in there, so you can directly try installing after
 those changes.