Add some documentation for mdbook test

book-example/src/cli/test.md

@@ -0,0 +1,9 @@
+# The test command
+
+It’s nice to be able to ensure that examples in documentation are current.
+mdBook supports a ‘test’ command, which will use Rustdoc to test out examples:
+
+```bash
+$ mdbook test
+[*]: Testing file: "/mdBook/book-example/src/README.md”
+```