doc: update README

README.md

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