Improve the contribution page (#29)

* Add the fork mechanism

* Update workflow

CONTRIBUTING.md

@@ -29,7 +29,9 @@ To create an issue, go to https://github.com/aboucaud/galcheat/issues/new, provi
 
 ## Pull-requests
 
-To add, update or correct information, the first step is to setup a development environment for `galcheat`
+To add, update or correct information, the first step is to setup a development environment for `galcheat` and then create feature branches for a given [pull-request](https://github.com/aboucaud/galcheat/pulls).
+
+> Note that you will have to [fork the project](https://guides.github.com/activities/forking/) to contribute.
 
 ### Getting a development environment (do once)
 
@@ -49,11 +51,33 @@ To add, update or correct information, the first step is to setup a development
     pre-commit install
     ```
 
+### Create a fork (do once)
+
+- [Fork the galcheat project](https://guides.github.com/activities/forking/)
+- Add the fork to the list of remote servers
+    ```sh
+    git remote add fork [email protected]:<your username>/galcheat.git
+    ```
+
 ### Code in a feature branch
 
-1. Don't forget to activate the development environment: `source dev-venv/bin/activate`.
-2. Create in a feature branch:
-    `git checkout -b <feat-branch>`.
-3. Commit your code into `<feat-branch>` (sometimes the commit will be rejected because of the `pre-commit` checks, just add the files a second time).
-4. Make sure to integrate the latest changes by regularly incorporating the latest work of the main branch into yours: `git rebase origin/main` and solve the conflicts, if any.
-5. Push your feature branch and create a pull-request.
+1. Activate the development environment
+    ```sh
+    source dev-venv/bin/activate
+    ```
+2. Create a feature branch (e.g. `featbranch` here)
+    ```sh
+    git checkout -b featbranch
+    ```
+3. Commit your code into `featbranch` (sometimes the commit will be rejected because of the `pre-commit` checks, just add the files a second time and commit again).
+4. Make sure to integrate the latest changes by regularly incorporating the latest work of the main branch into yours
+    ```sh
+    git rebase origin/main
+    ```
+    and solve the conflicts, if any.
+5. Push your feature branch to your fork
+    ```sh
+    git push -u fork featbranch  # the first time to set the target remote
+    git push                     # afterwards
+    ```
+6. Create [the pull-request](https://github.com/aboucaud/galcheat/pulls) and iterate from 3. until it is merged.