[guides] Add info about CHANGELOGs to contributing guide

guides/source/contributing_to_ruby_on_rails.textile

@@ -359,6 +359,31 @@ Rails follows a simple set of coding style conventions.
 
 The above are guidelines -- please use your best judgment in using them.
 
+h4. Updating the CHANGELOG
+
+The CHANGELOG is an important part of every release. It keeps the list of changes for every Rails version.
+
+You should add an entry to the CHANGELOG of the framework that you modified if you're adding or removing a feature, commiting a bug fix or adding deprecation notices. Refactorings and documentation changes generally should not go to the CHANGELOG.
+
+A CHANGELOG entry should summarize what was changed and should end with author's name. You can use multiple lines if you need more space and you can attach code examples indented with 4 spaces. If a change is related to a specific issue, you should attach issue's number. Here is an example CHANGELOG entry:
+
+<plain>
+*   Summary of a change that briefly describes what was changed. You can use multiple
+    lines and wrap them at around 80 characters. Code examples are ok, too, if needed:
+
+        class Foo
+          def bar
+            puts 'baz'
+          end
+        end
+
+    You can continue after the code example and you can attach issue number. GH#1234
+
+    * Your Name *
+</plain>
+
+Your name can be added directly after the last word if you don't provide any code examples or don't need multiple paragraphs. Otherwise, it's best to make as a new paragraph.
+
 h4. Sanity Check
 
 You should not be the only person who looks at the code before you submit it. You know at least one other Rails developer, right? Show them what you’re doing and ask for feedback. Doing this in private before you push a patch out publicly is the “smoke test” for a patch: if you can’t convince one other developer of the beauty of your code, you’re unlikely to convince the core team either.