Remove Changelog section from doc/contributing.rdoc [ci skip]

Replace it with a section on commit message formatting.  Also,
move the section on rebasing to the bottom, since that only
applies to committers and not most contributors.

Fixes [Bug #14886]

doc/contributing.rdoc

@@ -380,48 +380,25 @@ Here are some general rules to follow when writing Ruby and C code for CRuby:
 * ABBRs should be all upper case.
 * Do as others do
 
-=== ChangeLog
+=== Commit messages
 
-Although not required, if you wish to add a ChangeLog entry for your change
-please note:
+When you're ready to commit:
 
-You can use the following template for the ChangeLog entry on your commit:
+   git commit path/to/files
 
-  Thu Jan  1 00:00:00 2004  Your Name  <yourmail@example.com>
+This will open your editor in which you write your commit message.
+Use the following style for commit messages:
 
-  	* filename (function): short description of this commit.
-  	  This should include your intention of this change.
-  	  [bug:#number] [mailinglist:number]
+* Use a succint subject line.
+* Include reasoning behind the change in the commit message, focusing on why
+  the change is being made.
+* Refer to redmine issue (such as Fixes [Bug #1234] or Implements
+  [Feature #3456]), or discussion on the mailing list
+  (such as [ruby-core:12345]).
+* For GitHub issues, use [GH-#] (such as [Fixes GH-234]).
+* Follow the style used by other committers.
 
-  	* filename2 (function2): additional description for this file/function.
-
-This follows {GNU Coding Standards for Change
-Logs}[http://www.gnu.org/prep/standards/html_node/Change-Logs.html#Change-Logs],
-some other requirements and tips:
-
-* Timestamps must be in JST (+09:00) in the style as above.
-* Two spaces between the timestamp and your name. Two spaces between
-  your name and your mail address.
-* One blank line between the timestamp and the description.
-* Indent the description with TAB. 2nd line should begin with TAB+2SP.
-* Write a entry (*) for each change.
-* Refer to redmine issue or discussion on the mailing list.
-* For GitHub issues, use [GH-#] (such as [Fixes GH-234]
-* One blank line between entries.
-* Do as other committers do.
-
-You can generate the ChangeLog entry by running <code>make change</code>
-
-When you're ready to commit, copy your ChangeLog entry into the commit message,
-keeping the same formatting and select your files:
-
-    git commit ChangeLog path/to/files
-
-In the likely event that your branch becomes outdated, you will have to update
-your working branch:
-
-    git fetch origin
-    git rebase remotes/origin/master
+=== Contributing your code
 
 Now that you've got some code you want to contribute, let's get set up to
 generate a patch. Start by forking the github mirror, check the {github docs on
@@ -453,3 +430,18 @@ You may use the {'git format-patch'}[https://git-scm.com/docs/git-format-patch]
 command to generate patch files to upload to redmine.  You may also use
 the {'git request-pull'}[https://git-scm.com/docs/git-request-pull] command for
 formatting pull request messages to redmine.
+
+=== Updating the official repository
+
+If you are a committer, you can push changes directly into the official
+repository:
+
+    git push origin your-branch-name:master
+
+However, it is likely will have become outdated, and you will have to
+update it.  In that case, run:
+
+    git fetch origin
+    git rebase remotes/origin/master
+
+and then try pushing your changes again.