Update some Nomulus documentation (#2970)

This doesn't update everything -- it leaves out some of the more
complicated changes (architecture, code-structure, configuration,
install, and proxy-setup). Those will require more complete rewrites, so
I'm punting them to a future PR.

docs/local-testing.md

@@ -0,0 +1,34 @@
+# Local Testing
+
+## Running a local development server
+
+Nomulus provides a `RegistryTestServer` that is a lightweight test server
+suitable for running local development. It uses local versions of all Google
+Cloud Platform dependencies when available. Correspondingly, it is primarily
+useful for doing web UI development (i.e. the registrar console). It allows you
+to update Typescript, HTML, and CSS and see the changes simply by refreshing the
+relevant page in your browser.
+
+In order to serve content locally, there are two services that must be run: *
+the `RegistryTestServer` to serve as the backing server * the Angular service to
+provide the UI files
+
+In order to do this in one step, from the `console-webapp` folder, run:
+
+```shell
+$ npm install
+$ npm run start:dev
+```
+
+This will start both the `RegistryTestServer` and the Angular testing service.
+Any changes to Typescript/HTML/CSS files will be recompiled and available on
+page reload.
+
+One it is running, you can interact with the console by going to
+`http://localhost:4200` to view the registrar console in a web browser. The
+server will continue running until you terminate the process.
+
+If you are adding new URL paths, or new directories of web-accessible resources,
+you will need to make the corresponding changes in `RegistryTestServer`. This
+class contains all the routing and static file information used by the local
+development server.