kevinAlbs has created PR #934: Improve README.md in mongo-rust-driver

Issue Text:

1. Summary

This PR includes improvements to README.md. This PR proposes removing version numbers from the `All Feature Flags` table.

Fixes are included in separate commits to help review.

1. Background & Rationale

Removing version numbers was motivated by observing out-dated versions. Example: the README lists `tokio` 1.0. Cargo.toml specifies `1.17.0`.

Attempting to use an older incompatible `tokio` version results in a build error. Example:

```
[dependencies.mongodb]
version = "2.6.0"
features = ["tokio-runtime"]

[dependencies]
tokio =

{ version = ">=1.0.0,<1.17.0", features = ["full"] }

```

Results in a build error:
```
% cargo build
Updating crates.io index
error: failed to select a version for `tokio`.
... required by package `follow-readme v0.1.0 (/Users/q/code/rust-bootstrap/use-rust-driver/follow-readme-async)`
versions that meet the requirements `>=1.0.0, <1.17.0` are: 1.16.1, 1.15.0, 1.14.1, 1.14.0, 1.13.1, 1.13.0, 1.12.0, 1.11.0, 1.10.1, 1.10.0, 1.9.0, 1.8.5, 1.8.4, 1.8.3, 1.8.2, 1.8.1, 1.8.0, 1.7.3, 1.7.2, 1.7.1, 1.7.0, 1.6.4, 1.6.3, 1.6.2, 1.6.1, 1.5.1, 1.5.0, 1.4.0, 1.3.0, 1.2.0, 1.1.1, 1.1.0, 1.0.3, 1.0.2, 1.0.1
```

Rather than update the versions, this PR proposes removing the versions from the README. Readers can refer to Cargo.toml to see the required versions.