DOC: Add Python 2 statement to README, reference maintenance branch in CONTRIBUTING

Author: effigies

CONTRIBUTING.md

@@ -85,6 +85,8 @@ If you get no errors, you're ready to submit your changes!
 It's a good practice to create [a new branch](https://help.github.com/articles/about-branches/)
 of the repository for a new set of changes.
 
+For Python 2.7-compatible fixes, the branch should start from the `maint/1.3.x` branch on the
+upstream repository.
 
 **4. Submit a [pull request][link_pullrequest].**
 

README.rst

@@ -77,6 +77,24 @@ Information specific to Nipype is located here::
 
     http://nipy.org/nipype
 
+Python 2 Statement
+------------------
+
+Python 2.7 reaches its end-of-life in January 2020, which means it will
+*no longer be maintained* by Python developers. `Many projects
+<https://python3statement.org/`__ are removing support in advance of this
+deadline, which will make it increasingly untenable to try to support
+Python 2, even if we wanted to.
+
+The final series with 2.7 support is 1.3.x. If you have a package using
+Python 2 and are unable or unwilling to upgrade to Python 3, then you
+should use the following `dependency
+<https://www.python.org/dev/peps/pep-0440/#version-specifiers>`__ for
+Nipype::
+
+    nipype<1.4
+
+Bug fixes will be accepted against the ``maint/1.3.x`` branch.
 
 Support and Communication
 -------------------------