Add doc on customizing installer

Missed out adding file in previous commit

docs/topic/customizing-installer.rst

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+=========================
+Customizing the Installer
+=========================
+
+The installer can be customized with commandline parameters. The default installer
+is executed as:
+
+.. code-block:: bash
+
+    curl https://raw.githubusercontent.com/yuvipanda/the-littlest-jupyterhub/master/bootstrap/bootstrap.py \
+     | sudo python3 - \
+       <parameters>
+
+This page documents the various options you can pass as commandline parameters to the installer.
+
+Adding admin users
+===================
+
+``--admin <username>`` adds user ``<username>`` to JupyterHub as an admin user.
+This can be repeated multiple times.
+
+For example, to add ``admin-user1`` and ``admin-user2`` as admins when installing, you
+would do:
+
+.. code-block:: bash
+
+    curl https://raw.githubusercontent.com/yuvipanda/the-littlest-jupyterhub/master/bootstrap/bootstrap.py \
+     | sudo python3 - \
+       --admin admin-user1 --admin admin-user2
+
+Installing python packages in the user environment
+==================================================
+
+``--user-requirements-txt-url <url-to-requirements.txt>`` installs packages specified
+in the ``requirements.txt`` located at the given URL into the user environment at install
+time. This is very useful when you want to set up a hub with a particular user environment
+in one go.
+
+For example, to install the latest requirements to run UC Berkeley's data8 course
+in your new hub, you would run:
+
+.. code-block:: bash
+
+    curl https://raw.githubusercontent.com/yuvipanda/the-littlest-jupyterhub/master/bootstrap/bootstrap.py \
+     | sudo python3 - \
+       --user-requirements-txt-url https://raw.githubusercontent.com/data-8/materials-sp18/master/requirements.txt
+
+The URL **must** point to a working requirements.txt. If there are any errors, the installation
+will fail.
+
+.. note::
+
+   When pointing to a file on GitHub, make sure to use the 'Raw' version. It should point to
+   ``raw.githubusercontent.com``, not ``github.com``.