New use examples (PyGithub#920)

This PR add the follow code examples

* Issue examples (get list, get specific issue, create issue)
* Milestone examples (get list, get specific milestone, create milestone)

Ref.: PyGithub#874

doc/examples.rst

@@ -6,4 +6,6 @@ Examples
   examples/MainClass
   examples/Repository
   examples/Commit
-  examples/PullRequest
+  examples/PullRequest
+  examples/Issue
+  examples/Milestone

doc/examples/Issue.rst

@@ -0,0 +1,58 @@
+Issues
+======
+
+Get issue
+---------
+
+.. code-block:: python
+
+    >>> repo = g.get_repo("PyGithub/PyGithub")
+    >>> repo.get_issue(number=874)
+	Issue(title="PyGithub example usage", number=874)
+
+Create issue
+------------
+
+.. code-block:: python
+
+    >>> repo = g.get_repo("PyGithub/PyGithub")
+    >>> repo.create_issue(title="This is a new issue")
+	Issue(title="This is a new issue", number=XXX)
+
+Create issue with body
+----------------------
+
+.. code-block:: python
+
+    >>> repo = g.get_repo("PyGithub/PyGithub")
+    >>> repo.create_issue(title="This is a new issue", body="This is the issue body")
+	Issue(title="This is a new issue", number=XXX)
+
+Create issue with labels
+------------------------
+
+.. code-block:: python
+
+    >>> repo = g.get_repo("PyGithub/PyGithub")
+    >>> label = repo.get_label("My Label")
+    >>> repo.create_issue(title="This is a new issue", labels=[label])
+	Issue(title="This is a new issue", number=XXX)
+
+Create issue with assignee
+--------------------------
+
+.. code-block:: python
+
+    >>> repo = g.get_repo("PyGithub/PyGithub")
+    >>> repo.create_issue(title="This is a new issue", assignee="github-username")
+	Issue(title="This is a new issue", number=XXX)
+
+Create issue with milestone
+---------------------------
+
+.. code-block:: python
+
+    >>> repo = g.get_repo("PyGithub/PyGithub")
+    >>> milestone = repo.create_milestone("New Issue Milestone")
+    >>> repo.create_issue(title="This is a new issue", milestone=milestone)
+	Issue(title="This is a new issue", number=XXX)

doc/examples/Milestone.rst

@@ -0,0 +1,42 @@
+Milestone
+==========
+
+Get Milestone list
+------------------
+
+.. code-block:: python
+
+    >>> repo = g.get_repo('PyGithub/PyGithub')
+    >>> open_milestones = repo.get_milestones(state='open')
+    >>> for milestone in open_milestones:
+    ...    print(milestone)
+    ... 
+    Milestone(number=1) 
+    Milestone(number=2) 
+
+Get Milestone
+-------------
+
+.. code-block:: python
+
+    >>> repo = g.get_repo('PyGithub/PyGithub')
+    >>> repo.get_milestone(number=1)
+    Milestone(number=1) 
+
+Create Milestone
+----------------
+
+.. code-block:: python
+
+    >>> repo = g.get_repo('PyGithub/PyGithub')
+    >>> repo.create_milestone(title='New Milestone')
+    Milestone(number=1) 
+
+Create Milestone with State and Description
+-------------------------------------------
+
+.. code-block:: python
+
+    >>> repo = g.get_repo('PyGithub/PyGithub')
+    >>> repo.create_milestone(title='New Milestone', state='open', description='Milestone description')
+    Milestone(number=1)