README: Add example of the calls plus maybe a section with shellharden --help output

[HenrikBengtsson]

Brilliant tool. Thanks. I just learned about it today, and I found it hard to figure out exactly what the tool was, or if there were multiple tools without downloading/installing it. I'd like to suggest adding a section to the README.md that shows some example calls, e.g.

$ shellharden script.sh

maybe immediately above each screenshot example.

Another thing that would help people "passing by" could be to add:

# Usage

` ```
$ shellharden --help
Shellharden: The corrective bash syntax highlighter.

Usage:
	shellharden [options] [files]
	cat files | shellharden [options] ''

Shellharden is a syntax highlighter and a tool to semi-automate the rewriting
of scripts to ShellCheck conformance, mainly focused on quoting.

The default mode of operation is like `cat`, but with syntax highlighting in
foreground colors and suggestive changes in background colors.

Options:
	--suggest         Output a colored diff suggesting changes.
	--syntax          Output syntax highlighting with ANSI colors.
	--syntax-suggest  Diff with syntax highlighting (default mode).
	--transform       Output suggested changes.
	--check           No output; exit with 2 if changes are suggested.
	--replace         Replace file contents with suggested changes.
	--                Don't treat further arguments as options.
	-h|--help         Show help text.
	--version         Show version.

The changes suggested by Shellharden inhibits word splitting and indirect
pathname expansion. This will make your script ShellCheck compliant in terms of
quoting. Whether your script will work afterwards is a different question:
If your script was using those features on purpose, it obviously won't anymore!

Every script is possible to write without using word splitting or indirect
pathname expansion, but it may involve doing things differently.
See the accompanying file how_to_do_things_safely_in_bash.md or online:
https://github.com/anordal/shellharden/blob/master/how_to_do_things_safely_in_bash.md
` ```

## Usage advice

Don't apply `--transform` blindly; code review is still necessary: A script that relies on unquoted behavior
(implicit word splitting and glob expansion from variables and command substitutions) …

which of course requires more work, since you need to remember to update it whenever --help output changes