Release v0.6.0 (#50)

* Update subxt and bump version

* Update CHANGELOG

* Update README

* Fail build.rs when error, add diagnostics

* Rename template Cargo.toml files so they are not excluded from the package

It appears any dir with a Cargo.toml (not included in the workspace) is excluded from the package.

* Reorder env vars for clarity

* Add mention of crates.io release in CHANGELOG

* Valid category slugs