chore(docs): mention 1.0 release in the readme (#862)

<img width="1750" height="822" alt="image"
src="https://github.com/user-attachments/assets/ba9f691b-79cf-4bc1-a3ad-cad641914241"
/>


Re #701.

Author: ishymko

README.md

@@ -36,7 +36,10 @@
 
 ## 🧩 Compatibility
 
-This SDK implements the A2A Protocol Specification [`v0.3.0`](https://a2a-protocol.org/v0.3.0/specification).
+This SDK implements the A2A Protocol Specification [`0.3`](https://a2a-protocol.org/v0.3.0/specification).
+
+> [!IMPORTANT]
+> There is an [**alpha version**](https://github.com/a2aproject/a2a-python/releases?q=%22v1.0.0-alpha%22&expanded=true) 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) versions. 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 |
 | :--- | :---: | :---: |