replace text with link to docs

CONTRIBUTING.rst

@@ -1,69 +1,12 @@
-Prerequisites
-=============
-
-The demo code is supported and known to work on **Ubuntu only** (even
-closely related `Debian is known to fail`_).
-
-Therefore, prerequisites for other platforms listed below are provided
-mainly for the `hacking`_ reference.
-
-In general:
-
-* `swig`_ is required for compiling `m2crypto`_
-* `augeas`_ is required for the ``python-augeas`` bindings
-
-.. _Debian is known to fail: https://github.com/letsencrypt/lets-encrypt-preview/issues/68
-
-Ubuntu
-------
-
-::
-
-    sudo apt-get install python python-setuptools python-virtualenv python-dev \
-                 gcc swig dialog libaugeas0 libssl-dev libffi-dev \
-                 ca-certificates
-
-.. Please keep the above command in sync with .travis.yml (before_install)
-
-Mac OSX
--------
-
-::
-
-    sudo brew install augeas swig
-
-
-Installation
-============
-
-::
-
-    virtualenv --no-site-packages -p python2 venv
-    ./venv/bin/python setup.py install
-    sudo ./venv/bin/letsencrypt
-
-
-Usage
-=====
-
-The letsencrypt commandline tool has a builtin help:
-
-::
-
-   ./venv/bin/letsencrypt --help
-
-
-.. _augeas: http://augeas.net/
-.. _m2crypto: https://github.com/M2Crypto/M2Crypto
-.. _swig: http://www.swig.org/
-
 .. _hacking:
 
 Hacking
 =======
 
 In order to start hacking, you will first have to create a development
-environment. Start by installing the development packages:
+environment. Start by `installing dependencies and setting up Let's Encrypt`_.
+
+Now you can install the development packages:
 
 ::
 
@@ -82,6 +25,7 @@ The following tools are there to help you:
 - ``./venv/bin/tox -e lint`` checks the style of the whole project,
   while ``./venv/bin/pylint --rcfile=.pylintrc file`` will check a single `file` only.
 
+.. _installing dependencies and setting up Let's Encrypt: https://letsencrypt.readthedocs.org/en/latest/using.html
 
 .. _coding-style:
 Coding style