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

R=qsr@chromium.org

Review URL: https://codereview.chromium.org/1353883002 .

README.md

diff --git a/README.md b/README.md
index cc2a421..a9dd13d 100644
--- a/README.md
+++ b/README.md
@@ -204,6 +204,12 @@
 You can also request more information on what the tool is doing for you by
 passing the `--verbose` flag.
 
+### Running a Sky app
+
+To run a **local build** of Sky with a **local build** of Mojo, follow steps
+described in the [sky config file](mojo/tools/configs/sky). Note that the
+version of Sky has to be compatible with the version of Mojo for this to work.
+
 ### <a name="debugging"></a>Debugging, tracing, profiling
 
 Devtools `mojo_debug` allows you to interactively inspect a running shell,

mojo/tools/configs/sky

diff --git a/mojo/tools/configs/sky b/mojo/tools/configs/sky
index 78d5465..9fd972c 100644
--- a/mojo/tools/configs/sky
+++ b/mojo/tools/configs/sky
@@ -12,7 +12,7 @@
 #    --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 .