Add comment on issue example (#2081)

Add an example showing how to comment on an existing issue.

doc/examples/Issue.rst

@@ -9,6 +9,16 @@ Get issue
     >>> repo = g.get_repo("PyGithub/PyGithub")
     >>> repo.get_issue(number=874)
 	Issue(title="PyGithub example usage", number=874)
+	
+Create comment on issue
+-----------------------
+
+.. code-block:: python
+
+    >>> repo = g.get_repo("PyGithub/PyGithub")
+    >>> issue = repo.get_issue(number=874)
+    >>> issue.create_comment("Test")
+    	IssueComment(user=NamedUser(login="user"), id=36763078)
 
 Create issue
 ------------
@@ -65,4 +75,4 @@ Close all issues
     >>> repo = g.get_repo("PyGithub/PyGithub")
     >>> open_issues = repo.get_issues(state='open')
     >>> for issue in open_issues:
-    ...     issue.edit(state='closed')
+    ...     issue.edit(state='closed')