add doc of best practices for writing plugins

I think this would probably be useful, keeping track of some best practices:

• Always write plugins as a function that takes an options object, for future-proofing.
• When accepting defaultBlock/Mark/etc. arguments, use the Node.create() and Node.createProperties() static methods for standardizing these, just like core does.
• When allowing customization of what changes will actually be applied to the document, use a change function with signature (change, ...args) like core does.

If anyone thinks of others, feel free to add them here so we don’t forget!

[ianstormtaylor]

@oyeanuj Ah yes agreed! Actually I realized that I want to change up how those work by creating a separate plugin soon called slate-only-in that you can wrap any other plugins with. That way it doesn’t have to be repeated in all plugins code bases

[ianstormtaylor]

Added to the Plugins guide!