Add installation instructions

These are basically the same as for Eve,
see https://docs.python-eve.org/en/stable/install.html .

Closes #198

docs/index.rst

@@ -10,6 +10,7 @@ Developer's Guide
 .. toctree::
     :maxdepth: 2
 
+    install
     tutorial
     trivial
     upgrading

docs/install.rst

@@ -0,0 +1,59 @@
+Installation
+============
+
+This part of the documentation covers the installation of Eve-SQLAlchemy. The
+first step to using any software package is getting it properly installed.
+
+Installing Eve-SQLAlchemy is simple with `pip
+<https://pip.pypa.io/en/stable/>`_:
+
+.. code-block:: console
+
+    $ pip install eve-sqlalchemy
+
+
+Development Version
+-------------------
+
+Eve-SQLAlchemy is actively developed on GitHub, where the code is `always
+available <https://github.com/pyeve/eve-sqlalchemy>`_. If you want to work with
+the development version of Eve-SQLAlchemy, there are two ways: you can either
+let `pip` pull in the development version, or you can tell it to operate on a
+git checkout. Either way, virtualenv is recommended.
+
+Get the git checkout in a new virtualenv and run in development mode.
+
+.. code-block:: console
+
+    $ git clone https://github.com/pyeve/eve-sqlalchemy.git
+    Cloning into 'eve-sqlalchemy'...
+    ...
+
+    $ cd eve-sqlalchemy
+    $ virtualenv venv
+    ...
+    Installing setuptools, pip, wheel...
+    done.
+
+    $ . venv/bin/activate
+    $ pip install .
+    ...
+    Successfully installed ...
+
+This will pull in the dependencies and activate the git head as the current
+version inside the virtualenv. Then all you have to do is run ``git pull
+origin`` to update to the latest version.
+
+To just get the development version without git, do this instead:
+
+.. code-block:: console
+
+    $ mkdir eve-sqlalchemy
+    $ cd eve-sqlalchemy
+    $ virtualenv venv
+    $ . venv/bin/activate
+    $ pip install git+https://github.com/pyeve/eve-sqlalchemy.git
+    ...
+    Successfully installed ...
+
+And you're done!