fix(doc) update building

README.md

 
 ## Building the sources
 
-NOTE: you need Node.js >= 12 and npm >= 6
+NOTE: you need Node.js >= 12 and npm >= 7
 
 To build the library, just type:
 ```
 npm install
+npm run build
 ```
 To lint:
 ```
 npm test
 ```
 if you need to rebuild lib-jitsi-meet.min.js
-
 ```
-npm run postinstall
+npm run build
 ```
 
 Both linting and units will also be done by a pre-commit hook.