Document when --always-copy was introduced (in PR #409)

[techtonik]

Manually port change from PR #662.

[BrownTruck]

Hello!

As part of an effort to ease the contribution process and adopt a more standard workflow virtualenv has switched to doing development on the master branch. However, this Pull Request was made against the develop branch so it will need to be resubmitted against master.

If you do nothing, this Pull Request will be automatically migrated by @BrownTruck for you.

If you would like to retain control over this pull request then you should resubmit it against the master branch, closing and referencing the original Pull Request.

If you choose to migrate this Pull Request yourself, here is an example message that you can copy and paste:

Manually port change from PR #662.

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*This was migrated from pypa/virtualenv#686 to reparent it to the ``master`` branch. Please see original pull request for any previous discussion.*

If this pull request is no longer needed, please feel free to close it.

[BrownTruck]

This Pull Request has been automatically migrated to #915 to reparent it to the master branch. Please see the new pull request for any new discussion.