Document MSRV in Cargo.toml

While support for this was added in a much later cargo release, unknown
keys are simply ignored in older versions.

Author: sdroege

Cargo.toml

@@ -12,6 +12,7 @@ A library to run the pkg-config system tool at build time in order to be used in
 Cargo build scripts.
 """
 keywords = ["build-dependencies"]
+rust-version = "1.30"
 
 [dev-dependencies]
 lazy_static = "1"