Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -26,6 +26,10 @@ to group your changes into a small number of commits which we can review one at
 When completing a pull request, we will generally squash your changes into a single commit. Please
 let us know if your pull request needs to be merged as separate commits.
 
+### Technical design practice
+
+We use the [PlantUML](https://github.com/plantuml/plantuml) generation tool to provide planning solution and clarity for the architecture design of Hydra Lab, and most of the designs are documented in `*.puml` files, and running the following gradle task will generate the UML images in folder [docs/images/UML](docs/images/UML).
+
 ## Coding style conventions
 
 Under most cases, new file/folder name should follow the [snake case](https://en.wikipedia.org/wiki/Snake_case) style patterns. A fully capitalized name should come with a reasonable justification.