Add short getting started instructions (#5)

README.md

@@ -12,6 +12,10 @@
 
 This repository contains a demo manuscript, intended as a playground for everyone to practice using Manubot.
 
+To edit the manuscript, make changes to the files in the [`content`](content) directory.
+You can edit files with the GitHub web interface by using the pencil icon, as shown in the [getting started video](https://manubot.org/docs/getting-started.html).
+Alternatively, if you have experience with Git and GitHub, you can fork this repository and make a traditional [pull request](https://help.github.com/en/articles/creating-a-pull-request).
+
 ## Manubot
 
 <!-- usage note: do not edit this section -->