fix: Improved readme and help text

[timhaines]

Reformats some wording, adds context around finalizing a build.

[djones]

@timhaines that bit of the docs is autogenerated by Oclif. To update the wording, you'll want to update the description for each command, e.g. https://github.com/percy/percy-agent/blob/master/src/commands/exec.ts#L7

This has an added bonus that you'll be fixing not only the readme but the help text for the CLI 😄

Note the readme won't autogenerate until the release flow is run and that only happens when a new version is shipped. Technically I think this PR title should start with docs: and that won't ship a new version. If you feel strongly that you want to ship this update to the readme soon, feel free to change this PR to have fix: at the front.

Side note: in the raw readme there's some magic comments like

<!-- commands -->
Command docs are generated into this space.
<!-- commandsstop -->

and so that's how the auto doc generation works.

[djones]

🍍 LGTM