* tracing: prepare to release 0.1.11

Added

- `Span::is_none` method (#475)
- `LevelFilter::into_level` method (#470)
- `LevelFilter::from_level` function and `From<Level>` impl (#471)
- Documented minimum supported Rust version (#482)

Fixed

- Incorrect parameter type to `Span::follows_from` that made it
  impossible to call (#467)
- Missing whitespace in `log` records generated when enabling the `log`
  feature flag (#484)
- Typos and missing links in documentation (#405, #423, #439)

Signed-off-by: Eliza Weisman <eliza@buoyant.io>