Document autotrim of whitespace with hyphen in heredoc

[syl20bnr]

trafficstars

This form is used in the new book Terraform: Up & Running but it seems that it is not documented. This PR mention this hyphen form in the README.

Would also need to be documented in Terraform documentation: https://www.terraform.io/docs/configuration/syntax.html

[mdaniel]

Ironically, I was just making that change to the file in terraform-website and came here looking for some supporting documentation I could link to. I now know why I didn't find it. :-)

There doesn't seem to be an issue for this change in that repo; since there is one here, should I go ahead and put the website change in PR, or wait for this issue to resolve first in order to have a better language template for the TF side?

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