symplify system source (#61)

Author: ecarriou

src/systems/README.md renamed to src/system/README.md

@@ -3,7 +3,7 @@
 System Runtime has got a customizable system. You can update all its components, models or behaviors to follow your needs. To do so:
 
 * open a version of [System Designer](https://designfirst.io/systemdesigner/),
-* import the system you want,
+* import the system,
 * edit the system,
 * export it,
 * save it in the same place and