Update README.md (opnsense#544)

Hint the use of sphinx autobuild in Readme.md. It's a great tool to see a live preview while writing docs and editing them. The instant feedback really helps to get work done faster.

Author: Monviech

README.md

@@ -61,3 +61,15 @@ make html
 ```
 
 (```make clean``` to flush)
+
+#### Live preview of HTML documents
+
+Use `sphinx-autobuild` to track `source` for changes and get a live preview served via ``http://localhost:8000``.
+
+```
+pip[3] install sphinx-autobuild
+```
+
+```
+sphinx-autobuild source html
+```