Add some documentation

Author: ammaraskar

README.md

@@ -1,2 +1,20 @@
 # sphinx-problem-matcher
-Github Action problem matcher for the Sphinx documentation tool
+
+This action adds a problem matcher that catches Sphinx warnings to show them
+neatly inside your pull request like this:
+
+![Example Screenshot](https://i.imgur.com/xkUMS1a.png)
+
+The common usage of this will be like so:
+
+```yaml
+    - uses: ammaraskar/sphinx-problem-matcher@master
+    - name: "Build docs"
+      run: |
+        cd docs
+        make html
+```
+
+That is, simply run this action before building your docs.
+
+**Note: this does not handle actually building your docs.**