Minor improvements to github-release script

Summary: Newline and print out the link to the release page for ease-of-use.

Test Plan: ./github-release.sh -a <asset dir> -o <oauth token file> -t v0.20.3 -d

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4196