Adds upgrade guide

Author: nunomaduro

README.md

@@ -13,7 +13,7 @@ This readme/code introduces the upcoming Algolia API Client Python v2, the next
 
 **Development Status**: 4 - Beta.
 
-**Changelog**: Coming soon.
+**Upgrade Guide**: [Upgrade Guide](https://gist.github.com/nunomaduro/ddc2364465314390c9cfa94482ad2774)
 
 You'd like to contribute? Before start, we want to let you know that your **feedback** is important to us! Please consider start using this `v2` today! Found a bug or see something that can improved? Report it here: [github.com/algolia/algoliasearch-client-python/issues](https://github.com/algolia/algoliasearch-client-python/issues).