[GITIAN] Pull gitian sources in our repo

Summary:
This will help adding features in before they are merged upstream, and
simplify setting up the builds.
Docs will be updated in follow ups diffs in order to not make review
difficult.

Notes:

• A notice has been added to the doc/README.md file to mention the license of gitian-builder.
• The .arclint file is updated to prevent linter failures on the contrib/gitian-builder directory.
• There are missing files (namely the target-bin directory. This is due to an upstream bug in the .gitignore file which will be fixed in a follow-up diff.

Test Plan:

arc lint

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6972