add basic prosemirror instructions

prose/README.md

@@ -0,0 +1,7 @@
+# Building
+
+* Run `npm install` to download dependencies.
+* Run `npm run build` to build a production script in `dist/` or run
+`npm run develop` to build and watch for changes. You can use `prose.html`
+to test your development changes.
+* Manually copy the file `prose.bundle.js` to `static/js/`. _To be automated_