Make explicit that you can't use a version of spring direct from github

Closes #277

Author: jonleighton

CONTRIBUTING.md

@@ -31,6 +31,15 @@ $ RAILS_VERSION="~> 3.2.0" rake test:acceptance
 The apps in `test/apps` will be named based on the rails version and the
 spring version.
 
+## Testing with your app
+
+You cannot link to a git repo from your Gemfile. Spring doesn't support
+this due to the way that it gets loaded (bypassing bundler for
+performance reasons).
+
+Therefore, to test changes with your app, run `rake install` to properly
+install the gem on your system.
+
 ## Submitting a pull request
 
 If your change is a bugfix or feature, please make sure you add to

README.md

@@ -34,6 +34,9 @@ Add spring to your Gemfile:
 gem "spring", group: :development
 ```
 
+(Note: using `gem "spring", git: "..."` *won't* work and is not a
+supported way of using spring.)
+
 It's recommended to 'springify' the executables in your `bin/`
 directory: