Add Tidelift template

README.rst

+.. image:: https://tidelift.com/badges/github/GROUP/PROJECT
+   :target: https://tidelift.com/subscription/pkg/pypi-PROJECT?utm_source=pypi-PROJECT&utm_medium=readme
+
+
+Security Contact
+================
+
+If you wish to report a security vulnerability, the public disclosure
+of which may exacerbate the risk, please
+`Contact Tidelift security <https://tidelift.com/security>`_,
+which will coordinate the fix and disclosure privately.

docs/_templates/tidelift-sidebar.html

+<h3 class="donation">Professional support</h3>
+
+<p>
+Professionally-supported {{ project }} is available with the
+<a href="https://tidelift.com/subscription/pkg/pypi-{{ project }}?utm_source=pypi-{{ project }}&utm_medium=readme">Tidelift Subscription</a>.
+</p>

docs/conf.py

+
+# Custom sidebar templates, maps document names to template names.
+templates_path = ['_templates']
+html_sidebars = {'index': 'tidelift-sidebar.html'}