Enhance documentation on how to specify multiple explict targets in the same build command

[ADTC]

For making Windows build, I wanted both portable and installer version in the same build step.

I didn't want to run the build step twice, once for each target because each time the build step executes it takes 6 minutes just to set everything up. (I'm not sure if I run the build step again in the same workflow it would re-use existing resources, thus won't take the 6 minutes to setup. I assume it would still do everything from scratch. Please correct me if I'm wrong.)

After testing (for which I wish the GitHub action validated early what was provided in the args option and failed already before installing all the dependencies and setting up - topic for another day), I found that it is possible simply by listing the targets in args separated by spaces. (You shouldn't separate by comma, it wouldn't work.)

      - name: Build App
        uses: samuelmeuli/action-electron-builder@v1
        with:
          args: 'portable:x64 nsis:x64'

I find that the documentation in both the Readme as well as the website doesn't mention this fact. Hope the documentation can be enhanced. Thank you 🙂