Add require-yield to eslint:recommended

The require-yield rule ensures that all generators use yield. Since this rule only applies when generators are used, there’s no harm in turning it on for everyone (if you don’t use generators, you’ll never see a problem…if you do, then this is a good thing to warn). This would involve:

• Update eslint.json to have require-yield as "error"
• Update 3.0.0 migration guide to mention require-yield
• Update rules index (rules/README.md)
• Update rule metadata in require-yield.js
• Update rule documentation in require-yield.md

[onlywei]

Is there any further discussion as to why this is now recommended? I believe this inconveniences users of koa and co. Especially in koa’s recommended “Hello World” example: http://koajs.com/

[mysticatea]

There seem to be enough 👍s 😄