Update "Installation" docs page (#944)

- List all optional dependencies
- Clarify when conda-forge is recommended

CC ocefpaf jhamman

Author: shoyer

doc/installing.rst

@@ -6,7 +6,7 @@ Installation
 Required dependencies
 ---------------------
 
-- Python 2.7, 3.3, 3.4 or 3.5.
+- Python 2.7, 3.3, 3.4 or 3.5
 - `numpy <http://www.numpy.org/>`__ (1.7 or later)
 - `pandas <http://pandas.pydata.org/>`__ (0.15.0 or later)
 
@@ -22,6 +22,8 @@ For netCDF and IO
 - `pydap <http://www.pydap.org/>`__: used as a fallback for accessing OPeNDAP
 - `h5netcdf <https://github.com/shoyer/h5netcdf>`__: an alternative library for
   reading and writing netCDF4 files that does not use the netCDF-C libraries
+- `pynio <https://www.pyngl.ucar.edu/Nio.shtml>`__`: for reading GRIB and other
+  geoscience specific file formats
 
 For accelerating xarray
 ~~~~~~~~~~~~~~~~~~~~~~~
@@ -39,31 +41,38 @@ For parallel computing
 For plotting
 ~~~~~~~~~~~~
 
-- `matplotlib <http://matplotlib.org/>`__: required for :ref:`plotting`.
+- `matplotlib <http://matplotlib.org/>`__: required for :ref:`plotting`
 - `cartopy <http://scitools.org.uk/cartopy/>`__: recommended for
-  :ref:`plot-maps`.
+  :ref:`plot-maps`
+- `seaborn <https://stanford.edu/~mwaskom/software/seaborn/>`__: for better
+  color palettes
 
 
 Instructions
 ------------
 
 xarray itself is a pure Python package, but its dependencies are not. The
-easiest way to get them installed is to use conda_. You can then install xarray
-with its recommended dependencies with the conda command line tool::
+easiest way to get everything installed is to use conda_. To install xarray
+with its recommended dependencies using the conda command line tool::
 
     $ conda install xarray dask netCDF4 bottleneck
 
 .. _conda: http://conda.io/
 
-.. note::
-    You might consider using the
-    `conda-forge <https://conda-forge.github.io/>`__ channel, as it has been
-    shown to be more up-to-date and to better handle package dependencies.
+We recommend using the community maintained
+`conda-forge <https://conda-forge.github.io/>`__ channel if you need difficult
+to build dependencies such as cartopy or pynio:
+
+    $ conda install -c conda-forge xarray cartopy pynio
+
+New releases may also appear in conda-forge before being updated in the default
+channel.
 
 If you don't use conda, be sure you have the required dependencies (numpy and
 pandas) installed first. Then, install xarray with pip::
 
     $ pip install xarray
 
 To run the test suite after installing xarray, install
-`py.test <https://pytest.org>`__ and run ``py.test xarray``.
+`py.test <https://pytest.org>`__ (``pip install pytest``) and run
+``py.test xarray``.