README.md: Restore Bintray references with guidance [ci skip]
README.md: Restore Bintray references with guidance [ci skip]

The official site serves to pick the current latest version,
while Bintray is useful to pick past versions or _specific_
versions, which is useful for packagers who need a stable
URL for a given version.

Ref: Closes https://github.com/curl/curl-for-win/issues/11

vszakats/harbour-deps,vszakats/harbour-deps