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

README.md

 Mojo
 ====
 
 Mojo is an effort to extract a common platform out of Chrome's renderer and
 plugin processes that can support multiple types of sandboxed content, such as
 HTML, Pepper, or NaCl.
 
 ## Set-up and code check-out
 
 The instructions below only need to be done once. Note that a simple "git clone"
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 By default, `mojo_run` uses mojo:kiosk_wm as the window
 manager. You can pass a different window manager url using the
 `--window-manager` flag to override this.
 
 For additional information on `mojo_run` refer to the built-in help and the
 [documentation](mojo/devtools/common/docs/mojo_run.md).
 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,
 collect performance traces and attach a gdb debugger.
 
 For additional information refer to the built-in help and the
 [documentation](mojo/devtools/common/docs/mojo_debug.md).
 
 ### Android set-up
 
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 making your first change: [C++ in Chromium
 101](http://dev.chromium.org/developers/cpp-in-chromium-101-codelab).
 
 To land a change after receiving LGTM:
 ```
 $ git cl land
 ```
 
 Don't break the build! Waterfall is here:
 http://build.chromium.org/p/client.mojo/waterfall