Link and Content error in README.md

[jerryshia]

Describe the issue:
Missing step to download repo code.

Suggestion:
Add step before step 1:

1.Run the following command to download the project code:

azd init -t john0isaac/simple-flask-app-terraform

Describe the issue:
In the README.md, when click Docker, the link leads to a 404 page.:

Expected behavior:
Clicking on Docker can successfully open the page.

@hemarina and @BigMorty for notification.

[eamonoreilly]

Fixed link to open docker site and updated instructions to use azd init -t to get the content locally and initialize. #23