Added manual test information to the readme file.

README.md

@@ -5,6 +5,16 @@ Warning: This is an experimental plugin that comes with no support, use it at yo
 
 This package contains a Mermaid feature for CKEditor 5.
 
+## CKEditor 5 Mermaid - Running manual test
+
+You can test all of the functionalities of the CKEdtitor 5 mermaid feature with the manual test sample.
+
+Having the build in place, it's enough to execute the following at the root of your local clone of the project:
+
+```
+yarn start
+```
+
 ## License
 
 See [LICENSE.md](LICENSE.md) file.