docs: update Compact formatter docs (tokio-rs#1926)

## Motivation

Currently, the RustDoc for the `format::Compact` formatter in
`tracing-subscriber` describes the output from the `master` (v0.2.x)
version of the formatter, not the version on the v0.1.x branch.

## Solution

This PR updates the documentation to describe the actual output format.
Also, I added an example of the formatter in the `examples` directory.

Closes tokio-rs#1909

examples/README.md

@@ -69,3 +69,9 @@ This directory contains a collection of examples that demonstrate the use of the
 [tasks]: (https://docs.rs/tokio/0.2.21/tokio/task/index.html)
 [tokio-proxy]: https://github.com/tokio-rs/tokio/blob/v0.1.x/tokio/examples/proxy.rs
 [echo]: https://github.com/hyperium/hyper/blob/0.12.x/examples/echo.rs
+[`fmt`]: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/fmt/index.html
+[`format::Full`]: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/fmt/format/struct.Full.html
+[`format::Pretty`]: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/fmt/format/struct.Pretty.html
+[`format::Compact`]: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/fmt/format/struct.Compact.html
+[`format::Json`]: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/fmt/format/struct.Json.html
+[`fmt::Layer`]: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/fmt/struct.Layer.html