docs: Simplify README.md, add help, downloads

docs: Simplify README.md, add help, downloads

Add more guidance to README.md, namely the use of `--help`.

Add a section on downloads, pointing to the official download page,
automated builds, and community-contributed builds.

Move the status badges to a new line, simplifying the header
appearance.

Clean up the URLs with Markdown references, add the "Windows: " text
to AppVeyor builds to distinguish it from Linux.  Travis CI
unfortunately does not allow for a similar customization.

Replace "--" with "–", a Unicode replacement that should match the
intent.

Replace ```triple-backticks``` with `single-backticks`; both function
correctly and the latter is less annoying to read as plain text.

Fix bullet list indentation.

See https://docs.travis-ci.com/user/status-images/
And https://www.appveyor.com/docs/status-badges/