[manifest] add a license field (#328)

In many cases a web application and web apps are available under a specific open source license.
It would be beneficial and helpful for collaboration on web applications if a license filed would exist within the manifest.

A very good definition of such a field using SPDX license identifiers can be found here:
https://docs.npmjs.com/files/package.json#license

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/328