Add direct link to documentation

[d-e-s-o]

Make the URL to the documentation of the crate known in the manifest so that it will be easier to click through to it from a crates.io search.

[jhpratt]

https://doc.rust-lang.org/cargo/reference/manifest.html#the-documentation-field

If no URL is specified in the manifest file, crates.io will automatically link your crate to the corresponding docs.rs page when the documentation has been built and is available

[d-e-s-o]

I don't know what that's supposed to mean, but if you check https://crates.io/search?q=time there is no direct link.

[jhpratt]

File an issue for that, then? I am 100% certain the lack of a field is correct here.