README: add FOSSA badge

[ci skip]

README.md

@@ -136,3 +136,5 @@ There are 5 ways to install Gogs:
 ## License
 
 This project is under the MIT License. See the [LICENSE](https://github.com/gogs/gogs/blob/master/LICENSE) file for the full license text.
+
+[![FOSSA Status](https://app.fossa.io/api/projects/git%2Bgithub.com%2Fgogs%2Fgogs.svg?type=small)](https://app.fossa.io/projects/git%2Bgithub.com%2Fgogs%2Fgogs?ref=badge_small)