Update README re:ruby version support

README.md

@@ -54,12 +54,18 @@ For more examples, you should check out the
 
 ## Ruby version support
 
-* `master` - MRI 2.7, 3.0, 3.1 (same support as Ruby)
+Every savon release is tested with contemporary supported versions of ruby. Historical compatibility information:
+
+* `master` - same support as Ruby
+* 2.15.x - MRI 3.0, 3.1, 3.2, 3.3
+* 2.13.x, 2.14.x - MRI 2.7, 3.0, 3.1
 * 2.12.x - MRI 2.2, 2.3, 2.4, 2.5
 * 2.11.x - MRI 2.0, 2.1, 2.2, and 2.3
 
 If you are running MRI 1.8.7, try a 2.6.x release.
 
+Most changes are not backported to older versions of savon, or unsupported ruby versions.
+
 ## Running tests
 
 ```bash