chore: remove tokio-trace, add "Related Projects" to README (#1221)

## Motivation

The `tokio-trace` and `tokio-trace-core` crates have been renamed to
`tracing` and `tracing-core`, and moved to their own repository
(`tokio-rs/tracing`).

## Solution

This branch removes `tokio-trace` and `tokio-trace-core` from the
`tokio` repository. In addition, I've added a "Related Projects" section
to the root README, which lists `tracing` (as well as  `mio`, and
`bytes`) as other libraries maintained by the Tokio project. I thought
that this would help folks looking for `tokio-trace` here find it in its
new home.

In addition, it changes `tokio` to depend on `tracing-core` rather than
`tokio-trace-core`.

Closes #1159

Signed-off-by: Eliza Weisman <[email protected]>

Cargo.toml

@@ -18,8 +18,6 @@ members = [
   "tokio-timer",
   "tokio-tcp",
   # "tokio-tls",
-  # "tokio-trace",
-  # "tokio-trace/tokio-trace-core",
   "tokio-udp",
   # "tokio-uds",
 ]