Docs: Update download page with conda instrustrctions

docs/download.rst

@@ -1,9 +1,29 @@
 Download Devito
 ===============
 
-To install Devito from Github run the following commands from your terminal:
+The recommended way to install Devito uses the Conda package manager
+for installation of the necessary software dependencies. Please
+install either Anaconda_ or Miniconda_ using the instructions
+provided at the download links.
+
+.. _Anaconda: https://www.continuum.io/downloads
+
+.. _Miniconda: https://conda.io/miniconda.html
+
+To install the editable development version Devito, including examples,
+tests and tutorial notebooks, please run the following commands:
 
 .. code-block:: shell
 
    git clone https://github.com/opesci/devito.git
-   cd devito && pip install --user -r requirements.txt
+   cd devito
+   conda env create -f environment.yml
+   source activate devito
+   pip install -e .
+
+If you don't want to use the Conda environment manager, Devito can
+also be installed directly from GitHub via pip:
+
+.. code-block:: shell
+
+   pip install --user git+https://github.com/opesci/devito.git