Expand the devtools/common readme.

mojo/devtools/common/README.md

Index: mojo/devtools/common/README.md
diff --git a/mojo/devtools/common/README.md b/mojo/devtools/common/README.md
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--- a/mojo/devtools/common/README.md
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@@ -2,8 +2,39 @@
 
 Unopinionated tools for **running**, **debugging** and **testing** Mojo apps.
 
+The repo consists of:
+
+ - **devtoolslib** - Python module containing the core scripting functionality
+   for running Mojo apps: shell abstraction with implementations for Android and
+   Linux and support for apptest frameworks
+ - executable scripts - example embedders of devtoolslib and other utils
+
+## Embedding devtoolslib
+
+As devtools carry no assumptions about build system or file layout being used,
+the primary way of using devtools now is to embed the functionality provided by
+**devtoolslib** in a thin wrapper script. For examples, one can refer to mojo's
+[shell
+runner](https://github.com/domokit/mojo/blob/master/mojo/tools/mojo_shell.py)
+and [apptest
+runner](https://github.com/domokit/mojo/blob/master/mojo/tools/apptest_runner.py).
+
+## Executable scripts
+
+The set of executable scripts is WIP. We currently offer:
+
+ - **debugger** - allowing to send commands to the mojo:debugger app running in
+   the shell, allowing e.g. to interactively start and stop tracing
+
 ## Install
 
 ```
 git clone https://github.com/domokit/devtools.git
 ```
+
+## File location
+
+The library is canonically developed [in the mojo
+repository](https://github.com/domokit/mojo/tree/master/mojo/devtools/common),
+https://github.com/domokit/devtools is a mirror allowing to consume it
+separately.