Add vim help about vim-polyglot

[jrwrigh]

~7 years ago, there was a request to add documentation to the plugin in #9. The main reason to not due that was the lack of documentation for syntax files and that most of documentation that exists documented functionality within a plugin directory.

Correct me if I'm wrong, but I'd say things have changed a decent bit in 7 years. This plugin is no longer limited to just the syntax files and even then many syntax files have associated options with them. I think it'd be a good idea to start incorporating the documentation into this plugin. Thoughts?

[sheerun]

Vim-polygot strips plugin directory from plugins and imports just syntax and indentation

[sheerun]

Also there's point in having no documentation even if there are some functions available by mistake - vim-polyglot is not meant to be used as something that can be configured. In principle it should just add syntax, indentation, maybe some language-related options and that's it. I don't want to add documentation because I don't want to give any assurances about these options. I can replace any plugin at any time, the only thing that's for sure is syntax and indentation :)

[sheerun]

Maybe we could consider adding some documentation about vim-polyglot itself like g:polyglot_disabled option and credits like in readme