Add the repository field to Cargo.toml (#496)

To allow [Crates.io](https://crates.io/) , [lib.rs](https://lib.rs/) and
the [Rust Digger](https://rust-digger.code-maven.com/) to link to it.
See [the
manifest](https://doc.rust-lang.org/cargo/reference/manifest.html#the-repository-field)
for the explanation.

chorus-macros/Cargo.toml

@@ -4,6 +4,7 @@ version = "0.3.0"
 edition = "2021"
 license = "MPL-2.0"
 description = "Macros for the chorus crate."
+repository = "https://github.com/polyphony-chat/chorus"
 
 [lib]
 proc-macro = true