Issue 6132: Update README with new public token (#6133)
Issue 6132: Update README with new public token (#6133)

Updates snapshot builds section with the new public token.
Mentions alternative option for pre-release artifacts.
Fixes a broken link to Quick Start page.

Signed-off-by: Igor Medvedev <a9c96b99f8507ca5460c9a52faea21b634a395cc@gmail.com>

pravega/pravega,pravega/pravega,pravega/pravega