Generate flat badge for fossa license

[tenchooo]

Use https://shields.io dynamic badge.

Here's the badge:

And here's the badge code:
[![FOSSA Status](https://app.fossa.io/api/projects/git%2Bgithub.com%2Fphonicmouse%2FSharpPaste.svg?type=shield)](https://app.fossa.io/projects/git%2Bgithub.com%2Fphonicmouse%2FSharpPaste?ref=badge_shield)