This adds a new `std` feature to chrono that is enabled by default. By
deactivating this feature via `default-features = false` you can now use
chrono in applications that don't use the standard library. The `serde`
feature is supported as well.
Resolves#336
And fail CI in the future if `make readme` isn't run when it is needed.
CC conversation in #185, where the README didn't get updated for a year after
lib.rs was improved.