README.md in rack-contrib-1.8.0 vs README.md in rack-contrib-2.0.0

@@ -59,9 +59,20 @@
 use Rack::MailExceptions
 
 run theapp
 ```
 
+#### Versioning
+
+This package is [semver compliant](https://semver.org); you should use a
+pessimistic version constraint (`~>`) against the relevant `2.x` version of
+this gem.
+
+This version of `rack-contrib` is only compatible with `rack` 2.x.  If you
+are using `rack` 1.x, you will need to use `rack-contrib` 1.x.  A suitable
+pessimistic version constraint (`~>`) is recommended.
+
+
 ### Testing
 
 To contribute to the project, begin by cloning the repo and installing the necessary gems:
 
     gem install json rack ruby-prof test-spec test-unit