Mention the local-build-of-Sky + local-build-of-Mojo config in README.

mojo/tools/configs/sky

 # This is a configuration file one can use to run Sky apps from a local Sky
 # checkout using a locally built Mojo shell. As Sky is a layer on top of Mojo,
 # this file does not really belong in the Mojo repo and could hopefully move to
 # sky_engine.
 
 # To use this you need to check out sky_engine
 # (https://github.com/domokit/sky_engine ) and build for Android.
 
 # Then, run the lengthy command-line:
 #
 #  mojo/devtools/common/mojo_run \
 #    --android \
 #    --config-file mojo/tools/configs/sky \
 #    --config-alias SKY_SRC=/path_to_your_checkout/sky_engine/src \
-#    "mojo:window_manager https://sky/examples/hello_world/lib/main.dart"
+#    --embed https://sky/examples/hello_world/lib/main.dart
 
 # The sky apps are served from the Sky checkout, so all of
 # sky/packages/sky/example is available under https://sky/examples .
 
 {
   'dev_servers': [
     {
       'host': 'https://sky/',
       'mappings': [
         ('packages/', [
@@ skipping 12 matching lines @@
       'mappings': [
         ('', ['@{BUILD_DIR}']),
       ],
     },
   ],
 
   'content_handlers': {
     'application/dart': 'https://sky/sky_viewer.mojo',
   }
 }