README: document adding a page

Signed-off-by: William Woodruff <william@trailofbits.com>

README.md

@@ -10,3 +10,10 @@ edit documents.
 
 Once it has started, navigate to [http://localhost:10000](http://localhost:10000)
 in your browser! Any edits made will render as you save your work.
+
+### Adding new pages
+
+To add a new page, you'll need to do two things:
+
+1. Create the page itself in the appropriate location under [`./docs`](./docs/).
+2. Add the page to the `nav` hierarchy in [`mkdocs.yml`](./mkdocs.yml).