docs: update readme style

Using MarkdownLint in VSCode, this file was showing many linter errors,
mostly around whitespace issues, but a few others that made it annoying
enough when looking at the file, it was inconsistent and difficult to
read.

Also handles a few existing notes about caching.

Refs serverless#412
Resolves serverless#420
Resolves serverless#424

Signed-off-by: Mike Fiedler <[email protected]>

Author: miketheman