doc: update CHANGELOG and README with supported branch info

to be explicit about supporting 1.5.x while Rails 6.1 is supported

Author: flavorjones

CHANGELOG.md

@@ -1,4 +1,13 @@
 ## 1.6.0.rc2 / 2023-05-24
+* Dependencies have been updated:
+
+  - Loofah `~>2.21` and Nokogiri `~>1.14` for HTML5 parser support
+  - As a result, required Ruby version is now `>= 2.7.0`
+
+  Security updates will continue to be made on the `1.5.x` release branch as long as Rails 6.1
+  (which supports Ruby 2.5) is still in security support.
+
+  *Mike Dalessio*
 
 * HTML5 standards-compliant sanitizers are now available on platforms supported by
   Nokogiri::HTML5. These are available as:

README.md

@@ -234,6 +234,15 @@ Or install it yourself as:
     $ gem install rails-html-sanitizer
 
 
+## Support matrix
+
+| branch | ruby support | actively maintained | security support                       |
+|--------|--------------|---------------------|----------------------------------------|
+| 1.6.x  | >= 2.7       | yes                 | yes                                    |
+| 1.5.x  | >= 2.5       | no                  | while Rails 6.1 is in security support |
+| 1.4.x  | >= 1.8.7     | no                  | no                                     |
+
+
 ## Read more
 
 Loofah is what underlies the sanitizers and scrubbers of rails-html-sanitizer.