docs: add FOSSA badge (#725)

Author: fgreinacher

README.md

@@ -3,6 +3,7 @@
 ![Continuous Integration](https://github.com/System-IO-Abstractions/System.IO.Abstractions/workflows/Continuous%20Integration/badge.svg)
 [![Codacy Badge](https://api.codacy.com/project/badge/Grade/2e777fa545c94767acccd6345b1ed9b7)](https://app.codacy.com/gh/System-IO-Abstractions/System.IO.Abstractions?utm_source=github.com&utm_medium=referral&utm_content=System-IO-Abstractions/System.IO.Abstractions&utm_campaign=Badge_Grade_Dashboard)
 [![Renovate enabled](https://img.shields.io/badge/renovate-enabled-brightgreen.svg)](https://renovatebot.com/)
+[![FOSSA Status](https://app.fossa.com/api/projects/git%2Bgithub.com%2FSystem-IO-Abstractions%2FSystem.IO.Abstractions.svg?type=shield)](https://app.fossa.com/projects/git%2Bgithub.com%2FSystem-IO-Abstractions%2FSystem.IO.Abstractions?ref=badge_shield)
 
 At the core of the library is `IFileSystem` and `FileSystem`. Instead of calling methods like `File.ReadAllText` directly, use `IFileSystem.File.ReadAllText`. We have exactly the same API, except that ours is injectable and testable.