Add Read the Docs config file and README badge.

.readthedocs.yaml

@@ -0,0 +1,15 @@
+# Required
+version: 2
+
+sphinx:
+   configuration: docs/conf.py
+
+formats:
+  - pdf
+
+python:
+  version: "3.7"
+  install:
+    - requirements: docs/requirements.txt
+    - method: setuptools
+      path: .

README.rst

@@ -4,18 +4,23 @@ circuitry
 
 Embedded domain-specific combinator library for assembling abstract definitions of logic circuits.
 
-|pypi| |actions| |coveralls|
+|pypi| |readthedocs| |actions| |coveralls|
 
 .. |pypi| image:: https://badge.fury.io/py/circuitry.svg
    :target: https://badge.fury.io/py/circuitry
    :alt: PyPI version and link.
 
+.. |readthedocs| image:: https://readthedocs.org/projects/circuitry/badge/?version=latest
+   :target: https://circuitry.readthedocs.io/en/latest/?badge=latest
+   :alt: Read the Docs documentation status.
+
 .. |actions| image:: https://github.com/nthparty/circuitry/workflows/lint-test-cover-docs/badge.svg
    :target: https://github.com/nthparty/circuitry/actions/workflows/lint-test-cover-docs.yml
    :alt: GitHub Actions status.
 
 .. |coveralls| image:: https://coveralls.io/repos/github/nthparty/circuitry/badge.svg?branch=main
    :target: https://coveralls.io/github/nthparty/circuitry?branch=main
+   :alt: Coveralls test coverage summary.
 
 Purpose
 -------