add psalm badge

README.md

@@ -1,5 +1,7 @@
 # Pheature Flags Toggle Core
 
+[![Type coverage][ico-psalm]][link-psalm]
+
 Pheature flags toggle core library, take a look to our [main package]() to see our roadmap and project status.
 
 ## Installation
@@ -22,3 +24,6 @@ Your contributions are always welcome! Please have a look at the [contribution g
 
 We really believe in the Open Source Software, we built our carers around it, and we feel that we need to return our
 knowledge to the community. For this reason we release all our packages under [BSD-3-Clause licence](./LICENSE.md). 
+
+[ico-psalm]: https://shepherd.dev/github/feature-flags/toggle-core/coverage.svg
+[link-psalm]: https://shepherd.dev/github/feature-flags/toggle-core/coverage.svg