doc(dev): update instruction for installing

docs/en/dev.md

@@ -4,9 +4,23 @@ This package is the the main SeaCMS package.
 
 ## Installation
 
- - copy this repository on the server
+ - install [`git`](https://git-scm.com/downloads)
- - copy the repository `seacms-manager` in a folder `seacms-manager` into the the folder of this curret repository
+ - install [`composer`](https://getcomposer.org/download/)
- - then start [`composer install`](https://getcomposer.org/) for this repository
+ - clone repository
+   ```bash
+   git clone https://git.defis.info/SeaCMS/seacms.git seacms
+   ```
+   _choose the folder name by replacing `seacms` in the command line by the wanted path
+   or (equivalent to `git clone`)
+   ```bash
+   mkdir seacms
+   cd seacms
+   git init
+   git remote add origin https://git.defis.info/SeaCMS/seacms.git
+   git fetch --all
+   git checkout master
+   ```
+ - then start `composer install`
 
 ## TODO