add stability plan to README

Author: ryanflorence

README.md

@@ -56,6 +56,29 @@ The UMD build is also available on [npmcdn](https://npmcdn.com):
 
 You can find the library on `window.ReactRouter`.
 
+### Versioning and Stability
+
+React Router follows semver to the best of our interpretation of it. We want React Router to be a stable dependency that’s easy to keep current. Here is our upgrading stategy for your apps.
+
+Assuming we are currently on version 1.0:
+
+1. 2.0 is fully backward compatible with 1.0 so you can upgrade, and then update your code incrementally.
+2. All deprecated 1.0 API usage warns to the console and links to an upgrade guide.
+3. 3.0 will remove 1.0 deprecations completely.
+4. 3.0 will be released no sooner than three months after 2.0. This gives an API, in the worst case scenario, six months of life if you’re staying perfectly up-to-date.
+5. Some codemods that will automatically update your code will be available at rackt/rackt-codemod
+
+> If it’s fully backwards compatible, why isn’t that a minor release?
+
+If we didn’t provide the backwards compatibility then you wouldn’t be asking this question — but then upgrading would break your app. We don’t want to break your app, we want smooth, incremental upgrades.
+
+In practice, this means you can:
+
+1. Upgrade from 1.0 to 2.0 and your app will still run.
+2. Update your code incrementally to the new API, and you have three months before the next release to do it.
+3. Run the codemods to make some of (2) automatic.
+4. If your code runs without warnings, you can repeat this list with version 3.0
+
 ### What's it look like?
 
 ```js