Document how to use context

[sandinmyjoints]

• Component or Package Name: jsx-email
• Component or Package Version: 2.0.11

Documentation Is:

• Missing
• Needed
• Confusing
• Not Sure?

Please Explain in Detail...

I wanted to use context, and had to look at #179 to realize I needed to import { createContext, useContext } from 'jsx-email'; instead of from react.

Your Proposal for Changes

Mention that to use context, you must import these from jsx-email. Could go in the FAQ, or maybe there's a better place for it.

[shellscape]

Great suggestion. Would you be up for writing what you'd like to see in a PR?