Fix a broken link which direct to CONTRIBUTORS.md

Misc fix in the README.md file

Signed-off-by: NMassimeddu Cireddu <massimeddu@gmail.com>

- Update **Step 2** sub steps with more information
- Reword the "should" to "must" in the instanceUrl description to make clear, that this must be set for custom domains.

closes #88

When you work using a fork the issue will be
associated with the origin project but your
pipeline will run in the context of the
fork project.

This commit allow user to configure a different
remote to watch their pipeline in order to
adapt to this workflow.

Ref #59

This allows users to choose the git remote of his Gitlab project.

If no setting is provided, the previous behavior with auto detection is
kept.

Closes #22

Closes #24

Closes #25