Point at the GitHub mirror and document that it is read-only

`Cargo.toml` named github.com/bryant/invoiceninjars, a guess from the
git author name that was never a real repository. It now points at the
mirror that actually exists, so the crates.io "Repository" link will
resolve.

Gitea is the source of truth and a push mirror copies every commit to
GitHub. Anything written to the GitHub copy is overwritten by the next
sync, so a merged pull request there would silently vanish. README and
GOALS.md say so, and direct issues and pull requests to Gitea.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Cargo.toml

@@ -9,7 +9,7 @@ edition = "2021"
 rust-version = "1.88"
 description = "An async Rust client for the Invoice Ninja v5 API"
 license = "MIT"
-repository = "https://github.com/bryant/invoiceninjars"
+repository = "https://github.com/awkawb1/invoiceninja-sdk"
 
 [dependencies]
 reqwest = { version = "0.12", default-features = false, features = ["json", "native-tls"] }