add statement about ending python 2.7 support (#527)

Author: wholmgren

README.md

@@ -31,7 +31,10 @@ Installation
 pvlib-python releases may be installed using the ``pip`` and ``conda`` tools.
 Please see the [Installation page](http://pvlib-python.readthedocs.io/en/latest/installation.html) of the documentation for complete instructions.
 
-pvlib-python is compatible with Python versions 2.7 and 3.4-3.7
+pvlib-python is compatible with Python versions 2.7 and 3.4-3.7.
+
+**Python 2.7 support will end on June 1, 2019**. Releases made after this
+date will require Python 3.
 
 
 Contributing

docs/sphinx/source/whatsnew/v0.6.0.rst

@@ -3,6 +3,10 @@
 v0.6.0 (___, 2018)
 ---------------------
 
+**Python 2.7 support will end on June 1, 2019**. Releases made after this
+date will require Python 3. (:issue:`501`)
+
+
 API Changes
 ~~~~~~~~~~~
 * pvlib python is changing a handful of function names. In general, functions