[github-release] Add optional param to specify release notes path

Summary:
The script breaks if the version bump has not been applied yet.
This decouples the need to bump the version number from releasing.

Test Plan:

git clone https://github.com/bitcoin-abc/bitcoin-abc.git
./github-release.sh -o <path/to/oauth-token> -t v0.20.6 -a <path/to/0.20.6-binaries/> -d  # fails
./github-release.sh -o <path/to/oauth-token> -t v0.20.6 -a <path/to/0.20.6-binaries/> -r ~/bitcoin-abc/doc/release-notes.md -d  # succeeds

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4433