docs not in sync with README

[nkconnor]

The main README points to the README here: https://github.com/poem-web/poem/blob/master/poem/README.md

However, https://docs.rs/poem/latest/poem/ is more up to date. For example, it shows the test feature.

Does this project use cargo-readme? Maybe it would be easier to just use the docs.rs version instead of keeping both in sync. Then the homepage can point to docs.rs version.