doc: update README

README.md

@@ -8,6 +8,8 @@ You can use Jitsi Meet API to create Jitsi Meet video conferences with a custom
 
 ## Building the sources
 
+NOTE: you need Node.js >= 12 and npm >= 6
+
 To build the library, just type:
 ```
 npm install
@@ -20,4 +22,10 @@ and to run unit tests:
 ```
 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.