doc: clarify required Node / npm versions

doc/manual-install.md

@@ -225,6 +225,8 @@ npm install
 make
 ```
 
+_NOTE: When installing on older distributions keep in mind that you need Node.js >= 10 and npm >= 6._
+
 Edit host names in `/srv/jitsi-meet/config.js` (see also the example config file):
 ```
 var config = {