Add prerelease warning to readme.md (#728)

It will be helpful to point users of the library to code examples that will work for them.

Author: xaocon

readme.md

@@ -1,5 +1,5 @@
 # Bincode
-
+### The following information is for the prerelease 2.x versions of this library. The stable branch can be found [here](https://github.com/bincode-org/bincode/tree/v1.x).
 <img align="right" src="./logo.svg" />
 
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