Add info for version 0.9.x in readme to avoid confusion

Signed-off-by: Yilun <[email protected]>

README.md

@@ -1,3 +1,8 @@
+**Note: The master branch is frozen at version 0.8.x to maintain compatibility
+with the public testnet. See [balance-dev
+branch](https://github.com/nknorg/nkn/tree/balance-dev) for latest code.
+Version 0.9.x will be released in separate branches named v0.9.x.**
+
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