Add README.md with attribution and build instructions

- Links to original jdk-sandbox repository and specific commit
- References the included PDF proposal document
- Documents the modifications made for JDK 24 backport
- Provides build instructions and API overview

Author: simbo1905

README.md

@@ -0,0 +1,46 @@
+# JSON Experimental - JDK 24 Backport
+
+This repository contains a backport of the experimental JSON API from the [jdk-sandbox project](https://github.com/openjdk/jdk-sandbox) to JDK 24.
+
+## Origin
+
+This code is derived from the official OpenJDK sandbox repository at commit [d22dc2ba89789041c3908cdaafadc1dcf8882ebf](https://github.com/openjdk/jdk-sandbox/commit/d22dc2ba89789041c3908cdaafadc1dcf8882ebf) ("Improve hash code spec wording").
+
+The original proposal and design rationale can be found in the included PDF: [Towards a JSON API for the JDK.pdf](Towards%20a%20JSON%20API%20for%20the%20JDK.pdf)
+
+## Modifications
+
+This is a simplified backport with the following changes from the original:
+- Removed StableValue optimizations
+- Removed value-based class annotations  
+- Basic implementation without performance optimizations
+- Compatible with JDK 24 instead of future JDK versions
+
+## Building
+
+Requires JDK 24 (Early Access). Build with Maven:
+
+```bash
+mvn clean compile
+mvn package
+```
+
+## License
+
+Licensed under the GNU General Public License version 2 with Classpath exception. See [LICENSE](LICENSE) for details.
+
+## API Overview
+
+The API provides immutable JSON value types:
+- `JsonValue` - Base type for all JSON values
+- `JsonObject` - JSON objects (key-value pairs)
+- `JsonArray` - JSON arrays
+- `JsonString` - JSON strings
+- `JsonNumber` - JSON numbers
+- `JsonBoolean` - JSON booleans (true/false)
+- `JsonNull` - JSON null
+
+Parsing is done via the `Json` class:
+```java
+JsonValue value = Json.parse(jsonString);
+```