Allow extensions to specify a different file than README to be displayed on the marketplace

[joaomoreno]

@davidanthoff says:

It would be great if I could specify the path to some alternative markdown file in the package.json for inclusion in the marketplace instead of the README.md content. Essentially, if that setting is there in package.json, don't include the content from README.md on the marketplace, but instead use the content of the file specified.

The reason is that I want the README.md to contain information that is really more meant for the devs of the extension: status badges for builds, instructions how to dev on the extension etc. But for the users of the extension I want different content to be shown in the marketplace.

[joaomoreno]

Want to attempt a PR? Should be straightforward enough.

[davidanthoff]

I'm afraid I'm maxed out on the open-source projects I can contribute, so don't expect a PR from my anytime soon :)

[shanalikhan]

Need this for Code Settings Sync also

[raix]

I might do a pr tomorrow unless others get there first

[meajinkya]

@raix any update here? :)

[raix]

@meajinkya unfortunately no - I'm likely not going to have time for it 😞