Add design diagram + minor documentation improvement.

espresso-jshell/README.md

@@ -75,7 +75,11 @@ For hybrid projects like this one, a clear boundary between host and guest code
 To communicate between host and guest worlds, an [adapter](/~https://github.com/mukel/graalvm-demos/blob/master/espresso-jshell/src/main/java/com/oracle/truffle/espresso/jshell/EspressoExecutionControl.java) was implemented in host Java that, via interop, forwards all methods calls to an Espresso guest object, taking care of converting all the arguments and return values, translating guest exceptions to host exceptions...  
 On the Espresso side, a guest [LocalExecutionControl](https://docs.oracle.com/en/java/javase/11/docs/api/jdk.jshell/jdk/jshell/execution/LocalExecutionControl.html) executes all the methods calls forwarded by the host adapter; any other guest `ExecutionControl` implementation could be used.
 
-`espresso-jshell` runs on a guest `LocalExecutionControl` instance, so it behaves similar to `jshell -execution local` and shall not be compared with the default `jshell` execution mode e.g. class redefinition is not supported in this mode, `jshell -execution local` does not support it either. It does not support other execution engines other than `-execution espresso`, which is our host-to-guest adapter, see [EspressoLocalExecutionControlProvider](/~https://github.com/graalvm/graalvm-demos/blob/master/espresso-jshell/src/main/java/com/oracle/truffle/espresso/jshell/EspressoLocalExecutionControlProvider.java).
+<p align="center">
+    <img src="./diagram.svg">
+</p>
+
+`espresso-jshell` runs partially on a guest `LocalExecutionControl` instance, so it behaves similar to `jshell -execution local` and shall not be compared with the default `jshell` execution mode e.g. class redefinition is not supported in this mode, `jshell -execution local` does not support it either. It does not support other execution engines other than `-execution espresso`, which is our host-to-guest adapter, see [EspressoLocalExecutionControlProvider](/~https://github.com/graalvm/graalvm-demos/blob/master/espresso-jshell/src/main/java/com/oracle/truffle/espresso/jshell/EspressoLocalExecutionControlProvider.java).
 
 Since the Java compiler is part of the host code compiled by `native-image`, dynamically loaded annotation processors are not supported. Annotation processors must be compiled-in AOT by `native-image`. But this limitation could be lifted: since the Java compiler provides an interface to implement annotation processors, it could be possible to run the Java compiler on the host and the dynamically loaded annotation processors on Espresso using the same idea we just described.