[Migrated] Update README.md to explain requirements of AWS cron format

[jneves]

Originally from: Miserlou/Zappa#2210 by philsheard

I've suggested a change to the docs to help people schedule tasks using the cron() format. I tried to deploy an (existing) task today and got an error because I wasn't using the mandatory question marks. This is tucked away in third party docs so I thought it useful to add here.

Description

Text changes to README.md

GitHub Issues

Miserlou/Zappa#2209

[monkut]

aws link to cron or rate syntax linked in readme.

closing.