Set rust-version in Cargo.toml (#288)

Cargo.toml

@@ -1,12 +1,13 @@
 [package]
 name = "string_cache"
 version = "0.8.8"  # Also update README.md when making a semver-breaking change
-authors = [ "The Servo Project Developers" ]
+authors = ["The Servo Project Developers"]
 description = "A string interning library for Rust, developed as part of the Servo project."
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/servo/string-cache"
-documentation = "https://docs.rs/string_cache/"
+documentation = "https://docs.rs/string_cache"
 edition = "2018"
+rust-version = "1.70.0"
 
 # Do not `exclude` ./string-cache-codegen because we want to include
 # ./string-cache-codegen/shared.rs, and `include` is a pain to use