Update README to indicate dev status

Author: khvzak

README.md

@@ -17,6 +17,8 @@
 [Benchmarks]: https://github.com/khvzak/script-bench-rs
 [FAQ]: FAQ.md
 
+## The main branch is the development version of `mlua`. Please see the [v0.11](https://github.com/mlua-rs/mlua/tree/v0.11) branch for the stable versions of `mlua`.
+
 `mlua` is a set of bindings to the [Lua](https://www.lua.org) programming language for Rust with a goal of providing a
 _safe_ (as much as possible), high level, easy to use, practical and flexible API.