Update the README file part about commit messages

Link to the official home of the developer certificate of origin and
clarify some details about commit messages, as discussed on the mailing
list.

Signed-off-by: Thiago Macieira <thiago@macieira.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

README

@@ -87,12 +87,12 @@ we will not accept them. This means adding a line that says
 you wrote the code and have the right to pass it on as an open source
 patch.
 
-See: http://gerrit.googlecode.com/svn/documentation/2.0/user-signedoffby.html
+See: http://developercertificate.org/
 
 Also, please write good git commit messages.  A good commit message
 looks like this:
 
-	Header line: explaining the commit in one line
+	Header line: explain the commit in one line (use the imperative)
 
 	Body of commit message is a few lines of text, explaining things
 	in more detail, possibly giving some background about the issue
@@ -103,13 +103,20 @@ looks like this:
 	74 characters or so. That way "git log" will show things
 	nicely even when it's indented.
 
+	Make sure you explain your solution and why you're doing what you're
+	doing, as opposed to describing what you're doing. Reviewers and your
+	future self can read the patch, but might not understand why a
+	particular solution was implemented.
+
 	Reported-by: whoever-reported-it
 	Signed-off-by: Your Name <youremail@yourhost.com>
 
 where that header line really should be meaningful, and really should be
 just one line.  That header line is what is shown by tools like gitk and
 shortlog, and should summarize the change in one readable line of text,
-independently of the longer explanation.
+independently of the longer explanation. Please use verbs in the
+imperative in the commit message, as in "Fix bug that...", "Add
+file/feature ...", or "Make Subsurface..."
 
 
 A bit of Subsurface history: