Add Best Practice Section for React in Gutenberg

[timwright12]

After speaking with members of the team, these are the basic areas we want to touch on (in no particular order):

• Following standard React practices
• Standardizing around ES6
• Integrating with production output
• All projects need to be able to handle Webpack
• Wp.element usage
• Higher-order components in Gutenberg

Please add more in the comments section so we can make sure it all gets into the documentation somehow.