README.md Usage section doesn't include "async" feature, describes async usage

[Shel-M]

README.md uses the following Cargo instruction

[dependencies]
rustyrepl = "0.2"

But async usage described in next section requires the "async" features.

[dependencies]
rustyrepl = { version="0.2", features = ["async"] }

If feature is not included, build produces the following error

error[E0195]: lifetime parameters or bounds on method `process_command` do not match the trait declaration
  --> example\src\main.rs:28:14
   |
28 |     async fn process_command(&self, command: Cli) -> anyhow::Result<()> {
   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetimes do not match method in trait