Document installation using pip and git

docs/index.rst

@@ -11,6 +11,7 @@ Contents:
 .. toctree::
    :maxdepth: 2
 
+   install
    cli
    usage
    api/gitlab

docs/install.rst

@@ -0,0 +1,21 @@
+############
+Installation
+############
+
+``python-gitlab`` is compatible with python 2 and 3.
+
+Use :command:`pip` to install the latest stable version of ``python-gitlab``:
+
+.. code-block:: console
+
+   $ pip install --upgrade python-gitlab
+
+The current development version is available on `github
+<https://github.com/gpocentek/python-gitlab>`__. Use :command:`git` and
+:command:`pip` to install it:
+
+.. code-block:: console
+
+   $ git clone https://github.com/gpocentek/python-gitlab
+   $ cd python-gitlab
+   $ python setup.py install