chore: fixes the CI status badge and updates the README.md file (#569)

* Update README.md

Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

* Apply Joshua's feedback

Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

* Update README.md

Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

---------

Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

README.md

@@ -1,10 +1,18 @@
 # go-tuf
 
-[![build](https://github.com/theupdateframework/go-tuf/workflows/build/badge.svg)](https://github.com/theupdateframework/go-tuf/actions?query=workflow%3Abuild) [![Coverage Status](https://coveralls.io/repos/github/theupdateframework/go-tuf/badge.svg)](https://coveralls.io/github/theupdateframework/go-tuf) [![PkgGoDev](https://pkg.go.dev/badge/github.com/theupdateframework/go-tuf)](https://pkg.go.dev/github.com/theupdateframework/go-tuf) [![Go Report Card](https://goreportcard.com/badge/github.com/theupdateframework/go-tuf)](https://goreportcard.com/report/github.com/theupdateframework/go-tuf)
+![ci](https://github.com/theupdateframework/go-tuf/actions/workflows/ci.yml/badge.svg) [![Coverage Status](https://coveralls.io/repos/github/theupdateframework/go-tuf/badge.svg)](https://coveralls.io/github/theupdateframework/go-tuf) [![PkgGoDev](https://pkg.go.dev/badge/github.com/theupdateframework/go-tuf)](https://pkg.go.dev/github.com/theupdateframework/go-tuf) [![Go Report Card](https://goreportcard.com/badge/github.com/theupdateframework/go-tuf)](https://goreportcard.com/report/github.com/theupdateframework/go-tuf)
 
 This is a Go implementation of [The Update Framework (TUF)](http://theupdateframework.com/),
 a framework for securing software update systems.
 
+## API instability
+
+**Note**: go-tuf's API is in flux and backwards compatibility with version 0.7.0 and earlier will _**not**_ be maintained.
+
+We recommend production users of go-tuf pin their dependency so that changes in main do not cause disruptions.
+
+For more details on the changes, and the rationale for making them, see [issue #485](https://github.com/theupdateframework/go-tuf/issues/485) and the v0.7.0 [release notes](https://github.com/theupdateframework/go-tuf/releases/tag/v0.7.0)
+
 ## Directory layout
 
 A TUF repository has the following directory layout: