Add note on 2nd edition

README.md

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+## IMPORTANT NOTE ON 2ND EDITION
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+The 2nd edition of *Terraform: Up & Running* will be available shortly! If you're reading an early copy of the new edition of the book, please head over to the [2nd-edition branch](https://github.com/brikis98/terraform-up-and-running-code/tree/2nd-edition), which has all the code updated to Terraform 0.12, tons of new code for the new chapters in the book, and so on. It's a work in progress, so think of this branch as a preview of what things will look like when all the updates are finished.
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 ## Quick start
 
 All the code is in the [code](/code) folder. It is organized by language (terraform, bash, ruby, etc) and within each