describe upgrade topic in Readme

README.md

@@ -21,7 +21,7 @@ Generated project works in iOS and Android devices.
 - Same codebase for iOS and Android
 - Figwheel used for REPL and live coding.
   - Works in iOS (tested using simulator).
-  - Works in Android (tested on real device, does not work on simulator yet)
+  - Works in Android (tested on real device, does NOT work in Android simulator yet)
   - You can reload app any time, no problem.
   - "Debug in Chrome" is not required anymore.
 - Optimizations :simple is used to compile "production" index.ios.js and index.android.js
@@ -116,6 +116,28 @@ It will clean and rebuild index.ios.js and index.android.js with `optimizations
 Having index.ios.js and index.android.js build this way, you should be able to
 follow the RN docs to proceed with the release.
 
+## Upgrading existing Re-Natal project
+Do this if you want to use newer version of re-natal.
+
+Commit or backup your current project, so that you can restore it in case of any problem ;)
+
+Upgrade re-natal npm package
+```
+$ npm upgrade -g re-natal
+```
+In root directory of your project run
+```
+$ re-natal upgrade
+```
+This will overwrite only some files which usually contain fixes in newer versions of re-natal,
+and are unlikely to be changed by the user. No checks are done, these files are just overwritten:
+  - files in /env directory
+  - figwheel-bridge.js
+
+To upgrade React Native to newer version please follow the official
+[Upgrading](https://facebook.github.io/react-native/docs/upgrading.html) guide of React Native.
+Re-Natal makes no changes to the files generated by react-native so the official guide should be valid.
+
 ## Problems with Android simulator
 Using Figwheel with android simulator is not working out of the box yet.
 Looks like the main reason for that is that requests to http://localhost:8081 fails because