Improve plugin documentation

[corro]

An attempt to improve the plugin documentation as a reaction to https://groups.google.com/d/topic/sensu-users/iq_7kdah6Rc/discussion.

I tried to follow https://github.com/GhostLyrics/sensu-plugin-doc-draft where possible, but I found the section Files to be redundant, so I removed it completely.

[cwjohnston]

Hi @corro, thanks for this awesome pull request; a huge improvement over the existing documentation!

Will you please update the install example to include the --plugin or -p flag? e.g.

sensu-install --plugin sensu-plugins-ntp

It's missing from the source template, and I've opened a pull request to fix it there.

[corro]

@cwjohnston, thanks for the hint, it's fixed now. I followed blindly the example in the draft, good to see it has been corrected there as well.