Add dir-dependency documentation

[bradlc]

As discussed in #1537 this pull request adds documentation for a new dir-dependency message type that can be used to signify that a plugin depends on the contents of a directory. This would be used in the same way as the dependency message but with a dir property instead of a file property:

result.messages.push({
  type: 'dir-dependency',
  plugin: 'postcss-import',
  dir: '/js',
  parent: result.opts.from
})

Runners should watch this directory for changes and rebuild the CSS when something changes.

This PR also adds some documentation for the existing dependency message to the plugin and runner guides, and clarifies the usage of dependency messages in the "Writing a PostCSS Plugin" documentation.

[mischnic]

Is a dir-dependency recursive (= are changes in subdirectories also watched)?

[bradlc]

Is a dir-dependency recursive (= are changes in subdirectories also watched)?

I would say so, yes. Do you think the documentation should mention that?

[ai]

Do you think the documentation should mention that?

It is better to mention.

[bradlc]

Thanks for the feedback @ai. I have made some changes 👍

[bradlc]

@mischnic I have opened a pull request for this over on the Parcel repo.