fix(Cargo.toml): add categories

Signed-off-by: Martin Kröning <[email protected]>

Author: mkroening

Cargo.toml

@@ -4,6 +4,7 @@ version = "0.5.2"
 authors = ["Philipp Oppermann <[email protected]>"]
 license = "MIT OR Apache-2.0"
 keywords = ["volatile"]
+categories = ["no-std", "no-std::no-alloc"]
 description = "Volatile wrapper types for raw pointers"
 documentation = "https://docs.rs/volatile"
 repository = "https://github.com/rust-osdev/volatile"