Documentation

[emilpriver]

Add a documentation for plugin so it is easier to work with the plugin.

Should include stuffs:

• How to implement preview with frameworks
• How the plugin works and what parameter it sends to the user.

[SmartyP]

There is a link in the readme to the following documentation, but it gives a 404 - does it need to be made public..? https://emilpriver.github.io/jamstack-preview-and-deployments