Add README.

README.md

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+# rules_sphinx
+
+Rules for building [Sphinx](https://www.sphinx-doc.org/en/master/) documentation with
+[Bazel](https://bazel.build/).
+
+## WORKSPACE
+To incorporate `rules_sphinx` into your project at the following to your `WORKSPACE` file.
+
+```Starlark
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+http_archive(
+    name = "rules_sphinx",
+    # See release page for latest version url and sha.
+)
+```
+
+## Rules
+
+* `sphinx_html_gen` - Generates HTML documentation into `[NAME]_html` directory.
+* `sphinx_view` - Given an HTML generator, create target to launch viewer.
+* `sphinx_html` - A macro that creates a `sphinx_html_gen` and an associated `sphinx_view` with the
+  `[NAME].view` verb.
+
+## Examples
+
+To build the test documentation use:
+
+```Shell
+bazel build @rules_sphinx//test:test_docs
+```
+
+To build and view test documentation use:
+
+```Shell
+bazel run @rules_sphinx//test:test_docs.view
+```