Improve readme

[dareid]

• Remove godoc badge
• Show output
• Show local + docker usage together
• Fix local install instructions (go install)
• Add contributing section
• Install before usage?
• Explain docker volume
• Exit code 0 and stderr+stdout behaviour
• Ordering
• Show help output? or include details of output formats

Others from https://gist.github.com/alcortesm/f45a5f6b5e1543be3f1f88dde43cdecd

Thanks @rossmcf and @alcortesm

[akavel]

Please also do explain what algorithm(s) are used for searching and identifying the licenses. It's quite important when evaluating various "license detectors", as licenses can be hidden in a lots of non-standard places, such as file headers, etc; also subdirectories can have different licenses, also I'd like to know if the tool detects non-standard licenses (e.g. commercial ones) to warn me about them, etc, etc. Otherwise it's "the tool detected something, but I don't know if/to what extent I can rely on that". For example, go-license-detector has a rather nice explanation in the readme, from which I can see that it seems to only focus on LICENSE file, ignoring any subdirectories IIUC. This clearly communicates to me its features and its limitations.