Provide example for Reference.log_append

I would have found this useful when trying to do reflog additions. It
might not be massively complex but examples always help.

docs/references.rst

@@ -27,6 +27,23 @@ The Reference type
 .. automethod:: pygit2.Reference.resolve
 .. automethod:: pygit2.Reference.log
 .. automethod:: pygit2.Reference.log_append
+
+   Example::
+
+      >>> branch = repository.lookup_reference("refs/heads/master")
+      >>> branch.target = another_commit.id
+      >>> committer = Signature('Cecil Committer', 'cecil@committers.tld')
+      >>> branch.log_append(another_commit.id, committer,
+                            "changed branch target using pygit2")
+
+   This creates a reflog entry in ``git reflog master`` which looks like::
+
+      7296b92 master@{10}: changed branch target using pygit2
+
+   In order to make an entry in ``git reflog``, ie. the reflog for ``HEAD``, you
+   have to get the Reference object for ``HEAD`` and call ``log_append`` on
+   that.
+
 .. automethod:: pygit2.Reference.get_object