Declare maintenance mode (#626)

From the conversation on #620, let's try to steer people towards orjson
and away from writing pull requests that we're too paranoid to accept.

README.md

@@ -17,6 +17,18 @@ Install with pip:
 python -m pip install ujson
 ```
 
+## Project status
+
+> [!WARNING]
+> UltraJSON's architecture is fundamentally ill-suited to making changes without
+> risk of introducing new security vulnerabilities. As a result, this library
+> has been put into a *maintenance-only* mode. Support for new Python versions
+> will be added and critical bugs and security issues will still be
+> fixed but all other changes will be rejected. Users are encouraged to migrate
+> to [orjson](https://pypi.org/project/orjson/) which is both much faster and
+> less likely to introduce a surprise buffer overflow vulnerability in the
+> future.
+
 ## Usage
 
 May be used as a drop in replacement for most other JSON parsers for Python: