add blurb about python2.7 being dead

Author: phobson

CONTRIBUTING.md

@@ -1,4 +1,9 @@
-Please refer to matplotlib's [Coding
-Guidelines](http://matplotlib.org/devel/coding_guide.html)
-and [Git Workflow]
-(http://matplotlib.org/devel/gitwash/development_workflow.html)
+## Work with `mpl-probscale` source code
+Please refer to matplotlib's [Coding Guidelines](http://matplotlib.org/devel/coding_guide.html).
+
+## Git Workflow
+Please refer to matplotlib's [Git Workflow](http://matplotlib.org/devel/gitwash/development_workflow.html).
+
+## Python 2.7
+We're currently running a limited test suite with Python 2.7.
+Once I encounter a reason to completely drop Python 2.7 support, I will.