docs: add version compatibility notice

README.rst

@@ -34,14 +34,34 @@ Coveralls for Python
     .. image:: https://img.shields.io/pypi/implementation/coveralls.svg?style=flat-square&label=Python%20Implementations
         :target: https://pypi.org/project/coveralls/
 
-`coveralls.io`_ is a service for publishing your coverage stats online. This package provides seamless integration with `coverage.py`_ (and thus ``pytest``, ``nosetests``, etc...) in your Python projects::
+`coveralls.io`_ is a service for publishing your coverage stats online. This
+package provides seamless integration with `coverage.py`_ (and thus ``pytest``,
+``nosetests``, etc...) in your Python projects::
 
     pip install coveralls
     coverage run --source=mypkg setup.py test
     coveralls
 
 For more information and usage instructions, see our `documentation`_.
 
+Version Compatibility
+---------------------
+
+As of version 2.0, we have dropped support for end-of-life'd versions of Python
+and particularly old version of coverage. Support for non-EOL'd environments is
+provided on a best-effort basis and will generally be removed once they make
+maintenance too difficult.
+
+If you're running on an outdated environment with a new enough package manager
+to support version checks (see `the PyPA docs`_), then installing the latest
+compatible version should do the trick. If you're even more outdated than that,
+please pin to ``coveralls<2``.
+
+If you're in an outdated environment and experiencing an issue, feel free to
+open a ticket -- but please mention your environment! I'm willing to backport
+fixes to the 1.x branch if need be.
+
 .. _coveralls.io: https://coveralls.io/
 .. _coverage.py: https://coverage.readthedocs.io/en/latest/
 .. _documentation: http://coveralls-python.readthedocs.io/en/latest/
+.. _the PyPA docs: https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires

setup.py

@@ -32,6 +32,7 @@
             'coveralls = coveralls.cli:main',
         ],
     },
+    python_requires='>= 3.5',
     install_requires=[
         'coverage>=4.1,<6.0',
         'docopt>=0.6.1',