docs: Missing documentation for how to use assets

[robinwassen]

The argument assets to specify the assets path is mentioned in the docs - but no instructions on what files the folder expected to contain.

There is a folder called template/assets with four different sample electron icons named SampleAppx.*.png. But it feels counterintuitive to create an asset folder for your app and name the icons SampleAppx.

Some kind of instructions with more details on how to use assets would be a great addition. 👍

[robinwassen]

Gave it a try - it worked to copy the template/assets folder, modify the icons and specify the path. Still could use some docs though.

[jameshfisher]

Indeed, the template has these "SampleAppx" filenames hard-coded ... https://github.com/felixrieseberg/electron-windows-store/blob/master/template/appxmanifest.xml#L30-L31