Add 'used by' section in readme (#2)

Author: milesfrain

README.md

@@ -106,3 +106,13 @@ npm run package
 ```
 
 This will bundle and minify the source code so it is available to the end user.
+
+## Used By
+
+These libraries and applications are examples of `setup-purescript` in action:
+
+- [halogen-formless](https://github.com/thomashoneyman/purescript-halogen-formless/blob/main/.github/workflows/ci.yml)
+- [halogen-hooks](https://github.com/thomashoneyman/purescript-halogen-hooks/blob/main/.github/workflows/ci.yml)
+- [halogen-realworld](https://github.com/thomashoneyman/purescript-halogen-realworld/blob/main/.github/workflows/ci.yml)
+- [slug](https://github.com/thomashoneyman/purescript-slug/blob/main/.github/workflows/ci.yml)
+- Add your project here