chore(docs): mention 1.0 release in the readme

Re #701.

Author: ishymko

README.md

@@ -38,6 +38,9 @@
 
 This SDK implements the A2A Protocol Specification [`v0.3.0`](https://a2a-protocol.org/v0.3.0/specification).
 
+> [!NOTE]
+> There is an **alpha version** available with support for both [`1.0`](https://a2a-protocol.org/v1.0.0/specification/) and [`0.3`](https://a2a-protocol.org/v0.3.0/specification) protocols. Development for this version is taking place in the [`1.0-dev`](https://github.com/a2aproject/a2a-python/tree/1.0-dev) branch, tracked in [#701](https://github.com/a2aproject/a2a-python/issues/701).
+
 | Transport | Client | Server |
 | :--- | :---: | :---: |
 | **JSON-RPC** | ✅ | ✅ |