changes caching guide to add note on weak etags

guides/source/caching_with_rails.md

@@ -521,6 +521,14 @@ class ProductsController < ApplicationController
 end
 ```
 
+### A note on weak ETags
+
+Etags generated by Rails are weak by default. Weak etags allow symantically equivalent responses to have the same etags, even if their bodies do not match exactly. This is useful when we don't want the page regenerated for minor changes in response body. If you absolutely need to generate a strong etag, it can be assigned to the header directly.
+
+```ruby
+  response.add_header "ETag", Digest::MD5.hexdigest(response.body)
+```
+
 References
 ----------