Add missing "mvn clean install"
Add missing "mvn clean install"

Add for the source installation the instruction 
'''
cd language
mvn clean install
'''
TelluIoT/ThingML,TelluIoT/ThingML,TelluIoT/ThingML,TelluIoT/ThingML,TelluIoT/ThingML,TelluIoT/ThingML