FIX: adding recurse submodule

Author: 9and3

doc/contribute.md

@@ -25,7 +25,7 @@ Follow these steps to contribute to the project:
 1. Fork the diffCheck repository by clicking the **Fork** button on the [diffCheck repository](https://github.com/diffCheckOrg/diffCheck). Clone the repository to your local machine:
 
     ```console
-    git clone https://github.com/YOUR_USERNAME/diffCheck.git
+    git clone --recurse-submodules https://github.com/YOUR_USERNAME/diffCheck.git
     cd diffCheck
     ```
 
@@ -49,4 +49,4 @@ Follow these steps to contribute to the project:
     git add .
     git commit -m "WIP: Add my feature"
     git push origin my-feature
-    ```
+    ```