Fix README.frontend.md

doc/README.frontend.md

@@ -1,6 +1,26 @@
 # Frontend Development (Javascript/Vue/CSS)
 
-ntopng is currently using npm to compile the frontend code (JS/CSS) under httpdocs/dist
+The client side of the ntopng frontend is based on html/javascript/css/vue. Part of this (old style) is generated by Lua script. Instead new pages are written in modern html/javascript/css/vue and must be compiled.
+Ntopng is currently using npm to compile the frontend code (JS/CSS) under httpdocs/dist.
+Foreach file modified in http_src you must recompile httpdocs/dist.
+
+## Requirements
+To compile the new modern frontend part written in http_src you must have nodejs >= 18.15.0.
+To install nodejs on Ubuntu run these command:
+```
+curl -fsSL https://deb.nodesource.com/setup_18.x | sudo -E bash - &&\
+sudo apt-get install -y nodejs
+```
+On others Linux distributions to install nodeJs you can:
+- download nodejs from https://nodejs.org/dist/v18.15.0/node-v18.15.0-linux-x64.tar.xz
+- decomprime nodejs tar with:
+```
+tar -xvf node-v18.15.0-linux-x64.tar.xz
+```
+- export path of node-v18.15.0-linux-x64/bin:
+```
+export PATH=$PATH:$(pwd)/node-v18.15.0-linux-x64/bin
+```
 
 ## Quick Start
 
@@ -11,13 +31,13 @@ npm install
 ```
 
 In order to automatically recompile the code (debug mode) whenever a change to the
-frontend source files has been detected:
+frontend source files under http_src/ has been detected:
 
 ```
-npm run watch
+npm run watch 
 ```
 
-In order to compile for production (ntopngjs bundle only):
+In order to compile frontend source files under http_src/ for production (ntopngjs bundle only):
 
 ```
 npm run build:ntopngjs
@@ -28,4 +48,3 @@ In order to compile for production (also third party code):
 ```
 npm run build
 ```
-