graphyte: Add document for plugin shell

Add some instructions to use the plugin shell.

BUG=none
TEST=none

Change-Id: I67643621df8495a3a08dec80878768c73aa61b94
Reviewed-on: https://chromium-review.googlesource.com/586207
Commit-Ready: Shen-En Shih <petershih@chromium.org>
Tested-by: Shen-En Shih <petershih@chromium.org>
Reviewed-by: Chih-Yu Huang <akahuang@chromium.org>
diff --git a/README.md b/README.md
index 27ab628..86943c7 100644
--- a/README.md
+++ b/README.md
@@ -37,3 +37,14 @@
 ## User manual
 Please find the user manual [here](https://groups.google.com/forum/#!topic/graphyte-public/GJgiDc482K8)
 for more details.
+
+## Interactive shell
+An interactive shell is also implemented to analyze the interactions
+between a DUT and an instrument step by step.
+
+Follow these steps to start the interactive shell:
+1. Install the graphyte framework and necessary plugins.
+2. Follow the user manual to create a valid config file.
+3. Run the interactive shell by:
+
+    $ python -m graphyte.plugin_shell path/to/config/file