Add a message around upgrading

Author: patch0

README.md

@@ -102,8 +102,6 @@ There is also a helper for the logout route:
 link_to 'Log out', rpi_auth_logout_path
 ```
 
-### Sin
-
 ### Redirecting users to the "next step"
 
 There are a three possible places the user will end up at following logging in,
@@ -152,6 +150,13 @@ class in `config/application.rb`.
 config.railties_order = [RpiAuth::Engine, :main_app, :all]
 ```
 
+## Upgrading between versions.
+
+This project follows semantic versioning, so upgrades between minor and patch
+versions should not need any code or configuration changes.
+
+For major version upgrades see the [upgrading docs](UPGRADING.md).
+
 ## License
 
 The gem is available as open source under the terms of the [MIT License](https://opensource.org/licenses/MIT).