add archival notice to README

This repository is being archived. Point readers at the standard-library
or community package that replaces it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -1,3 +1,21 @@
+> [!IMPORTANT]
+> **This repository is archived and no longer maintained.**
+>
+> These functions are one-line wrappers around the standard library.
+>
+> **Use instead** — standard library:
+> ```go
+> import (
+>     "crypto/sha256"
+>     "fmt"
+> )
+>
+> sum := fmt.Sprintf("%x", sha256.Sum256([]byte(value))) // hex-encoded SHA256
+> ```
+> Use `crypto/sha512` the same way for SHA512.
+
+---
+
 # Go Hash
 
 Simple wrapper functions which take arbitrary string input and return string hashes in hex encoding.