add rust-version requirement to Cargo.toml

Author: almindor

Cargo.toml

@@ -8,6 +8,7 @@ description = "HAL for the E310x family of microcontrollers."
 keywords = ["riscv", "e310", "hal"]
 license = "ISC"
 edition = "2018"
+rust-version = "1.59"
 
 [dependencies]
 embedded-hal = { version = "0.2.6", features = ["unproven"] }