add info about grid generation command

INSTALL.md

@@ -20,6 +20,12 @@ PWD=rootpassword make install
 * if you let default values, MW should be available at localhost:666 and the phpmyadmin at localhost:8080
 * You'd better leave default values, some stuff is hardcoded...
 
+* Once application installed, you can generate some grid with these commands in the php container. It will generates 10 grids in french and english with at least 150 (and 250 for french) foundable forms. 
+
+> php bin/console magicword:generate-grid english 10 150
+> php bin/console magicword:generate-grid french 10 250
+
+
 # Update
 ```
 git pull origin master