chore: adding document link to CONTRIBUTING.md

CONTRIBUTING.md

@@ -19,6 +19,7 @@ Please follow these guidelines when contributing to the project:
 You can use these commands in the Makefile:
 
 - `make build` compiles the code into executable binaries.
+- `make build_gui` compiles the gtk GUI code into executable binary.
 - `make devtools` installs required development tools.
 - `make fmt` formats the code according to the Go standards.
 - `make check` runs checks on the code, including formatting and linting.
@@ -27,6 +28,10 @@ You can use these commands in the Makefile:
 - `make proto` generates [protobuf](https://protobuf.dev/) files.
   Run this target if you have made any changes to the proto buffer files.
 
+### GUI Development
+
+Development of the Pactus Core GUI have some requirements on your machine which you can find a [quick guide about it here](./docs/gtk-gui-development.md).
+
 ### Error and Log Messages
 
 Error and log messages should not start with a capital letter (unless it's a proper noun or acronym) and
@@ -115,4 +120,4 @@ Please read it before contributing to the project.
 
 ---
 
-Thank you for your contributions to Pactus blockchain!
+Thank you for your contributions to the Pactus blockchain!