Set placeholders in Windows README.txt for installer, getdns and Stubby versions, and add OpenSSL version.
Set placeholders in Windows README.txt for installer, getdns and Stubby versions, and add OpenSSL version.

This file is installed as part of the Windows package. Rather than update this file manually on every release, add placeholders that the package release process can replace mechanically.

getdnsapi/stubby,getdnsapi/stubby