Create contributing.md

Author: ChenZiHong-Gavin

.github/contributing.md

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+## Contribution Guide
+Here are the steps to contribute to this project:
+
+1. Star this repository.
+2. Fork this repository.
+   
+   Type the following command on Git bash console:
+   ```bash
+   git clone https://github.com/open-sciencelab/GraphGen.git
+   ```
+
+3. Create a new branch
+   
+    Now before making changes to the files, go to your terminal under the repo you just cloned, and type the following:
+    
+    ```bash
+    git checkout -b add-my-name
+    ```
+  
+    By running the above command, you just created a new branch called add-my-name and checked it out, what this does is that it creates a new branch with the commit history of the master branch or the branch that you were on previously.
+
+4. Make your changes and push your code.
+
+   ```
+   git add .
+   git commit -m "xxx"
+   git push
+   ```
+
+   This will create a new commit with the changes you made.
+
+5. Now create a pull request and add the title.
+
+     Sit back and relax while your pull request is being reviewed and merged.