flatpak-node-generator README: Mention XDG path of electron node headers

[threema-danilo]

I had to dig in the source code to find out that the path is different when --xdg-layout is set.

[MrMYHuang]

Thank you. It works.
However, by my test, the ln command line should be run before npm install --offline. Otherwise, electron-builder will try to get Electron headers from Internet and fail with these errors:

    gyp http GET https://electronjs.org/headers/v16.2.3/node-v16.2.3-headers.tar.gz
    gyp WARN install got an error, rolling back install
    gyp ERR! configure error 
    gyp ERR! stack Error: getaddrinfo EAI_AGAIN electronjs.org
    gyp ERR! stack     at GetAddrInfoReqWrap.onlookup [as oncomplete] (dns.js:71:26)

It could be related to https://www.electronjs.org/docs/latest/tutorial/using-native-node-modules

# Install all dependencies, and store cache to ~/.electron-gyp.
HOME=~/.electron-gyp npm install

This is my flatpak manifest for reference:
https://github.com/flathub/io.github.mrmyhuang.cbetar2/blob/7c1e6c0c3f0084574398a9d17109630f28ba007e/io.github.mrmyhuang.cbetar2.yml

[MrMYHuang]

However, by my test, the ln command line should be run before npm install --offline.

Sorry, this was wrong. I thought it was so, because my npm install --offline triggered the package.json postinstall script to rebuild native module by electron-builder. This sentence is not true if removing the postinstall script.

Actually, by me tests, the ln command line shall be run before electron-builder rebuilds native modules. Not releated to run before or after npm install --offline.