Update installation instructions

[verythorough]

For greater simplicity and consistency with Netlify users’ existing configuration files, we’re deprecating experimental support for Netlify config files in YAML or JSON (netlify/build#975).

To get new plugin users started on the right foot, I’m proposing updates for all plugin READMEs to use netlify.toml format in the installation instructions.

This docs-only change should require no updates to your plugin code.

I've also made the following updates to reflect the current plugins API:

• Replaced type with package, and config with inputs.
• Removed the npm install step. This is no longer necessary.

[verythorough]

@Munter 👋 Checking in on the status of this PR.

We're now displaying plugins in the Netlify UI, using the list at https://github.com/netlify/plugins/blob/master/plugins.json. In order to ensure that all plugins in the UI are functional and match the documented configuration methods, I'll be making a PR tomorrow to remove this plugin from the list.

We'll be happy to add it back when this PR is merged and you can confirm that the plugin still works in the Netlify Build Plugins Beta!

[Munter]

Thank you for the updates

[verythorough]

Thank you!