doc(dropbox): Add documentation for the mobile app.

doc/mobile-dropbox.md

@@ -0,0 +1,28 @@
+# Setting up Dropbox integration
+1. Create a Dropbox app.
+2. Add the following to ```ios/app/src/Info.plist``` by replacing `<APP_KEY>`
+   with your own Dropbox app key (which can be found in the
+   [App Console](https://www.dropbox.com/developers/apps)):
+```
+<key>CFBundleURLTypes</key>
+<array>
+  <dict>
+    <key>CFBundleURLName</key>
+    <string></string>
+    <key>CFBundleURLSchemes</key>
+    <array>
+      <string>db-<APP_KEY></string>
+    </array>
+  </dict>
+</array>
+<key>LSApplicationQueriesSchemes</key>
+<array>
+  <string>dbapi-2</string>
+  <string>dbapi-8-emm</string>
+</array>
+```
+
+**NOTE:** Both Android and iOS builds of the apps will parse the Dropbox app key
+from ```ios/app/src/Info.plist```.
+
+**NOTE:** See [Dropbox developer guide](https://www.dropbox.com/developers/reference/developer-guide) for more information