Add upstream maintenance notice (#132)

* Update README.md with Upstream Maintenance Notice

* Apply suggestions from code review

Co-authored-by: Thomas Honeyman <[email protected]>

---------

Co-authored-by: Thomas Honeyman <[email protected]>

Author: dragospe

README.md

@@ -19,6 +19,14 @@ The source code currently covers Halogen 7 with PureScript 0.15, but there are v
 
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+# ⚠️ Upstream Maintenance Notice (Spring 2025)
+
+This repository provides a PureScript frontend that was intended to point to a spec-compliant, public, RealWorld backend. 
+
+However, the upstream RealWorld project is [no longer maintained](https://github.com/gothinkster/realworld/issues/1611) and the public API service implementing the RealWorld spec is no longer operational. The instructions below _will not work_ by default.
+
+If you wish to proceed, you will need to work out another option for accessing a RealWorld API (such as self-hosting locally). You can consider using one of the [RealWorld backends](https://codebase.show/projects/realworld?category=backend).
+
 ## Installation
 
 First, clone the repository: