Add linkage to codemods. 🔬

Author: timdorr

upgrade-guides/v2.0.0.md

@@ -24,6 +24,12 @@ We have done our best to provide backwards compatibility with deprecated APIs. I
 
 The deprecation warnings should also lead you to the relevant part of this document. If one doesn't, please open a pull request with a fix. Also, if any part of this document could be improved, please let us know how. Confound it, our bias is often inescapable!
 
+### Upgrade with Codemods
+
+Using a tool called [jscodeshift](https://github.com/facebook/jscodeshift), we have made available some codemods for upgrading your code to the new APIs automatically: https://github.com/rackt/rackt-codemod
+
+A codemod is much like Babel, but instead of converting your ES2015 code to ES5 compatible syntax, it does a limited set of transformations on function names, arguments, common patterns and more. One way to think of jscodeshift (the underlying tool) is "jQuery for code". These codemods aren't bulletproof, so be sure to test your code after you run them. But they can help with upgrading a large codebase to remove deprecation warnings you're now triggering.
+
 ## Using `history` with `Router`
 
 ### History singletons provided