Add docs explaining how to test the client on a real mobile device

Testing the client on a mobile device currently requires several steps
which will be non-obvious to new developers. Add documentation to guide
users through the process.

docs/developers/index.rst

@@ -8,6 +8,7 @@ the Hypothesis client.
    :maxdepth: 2
 
    developing
-   security
    envvars
+   mobile
+   security
    arch/index

docs/developers/mobile.md

+# Mobile Development
+
+## Testing the client on a mobile device
+
+If you have made changes to the client that could affect the mobile experience, it is a good idea to test them on a real device. Such changes should ideally be tested with at least current versions of iOS Safari and Chrome for Android.
+
+1. Make sure your development system and mobile device are on the same local network.
+1. Get the IP address or host name of your development system (`$HOSTNAME` in the steps below). You can do this using the `hostname` terminal command on Mac/Linux.
+1. Configure the "h" service to allow incoming connections from other systems by editing `conf/development-app.ini` and changing the `host` setting from `localhost` to `0.0.0.0`.
+1. Configure the "h" service to load the client from this host and start the dev
+   server:
+   ```sh
+   # In the "h" repository
+
+   # Configure the URL that the client is loaded from in pages
+   # that embed Hypothesis
+   export CLIENT_URL=http://$HOSTNAME:3001/hypothesis
+
+   make dev
+   ```
+1. Configure the client to load assets from this hostname and start the dev
+   server:
+   ```sh
+   # In the "client" repository
+
+   # Set URL which sidebar app ("app.html") is loaded from
+   export H_SERVICE_URL=http://$HOSTNAME:5000
+   # Set hostname used when generating client asset URLs
+   export PACKAGE_SERVER_HOSTNAME=$HOSTNAME
+
+   gulp watch
+   ```
+   When `gulp watch` runs, it will print out the URLs used for the "h" service and client assets. These should include `$HOSTNAME` instead of `localhost`.
+1. On your mobile device, go to a page which has the client embedded such as `http://$HOSTNAME:3000` or `http://$HOSTNAME:5000/docs/help`.