Fix MSRV in README.

Author: matoushybl

README.md

@@ -58,9 +58,8 @@ If you are unfamiliar with embedded development using Rust, there are a number o
 Minimum supported Rust version
 ------------------------------
 
-The minimum supported Rust version at the moment is **1.39.0**. Older versions
-**may** compile, especially when some features are not used in your
-application.
+We have decided not to yet commit to a specific minimum supported Rust version as this HAL is still under development and committing to MSRV would be premature.
+Once the HAL stabilizes, a proper MSRV will be published.
 
 Changelog
 ---------