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
licence of `gitian-builder`.
- The `.arclint` file is updated to prevent linter failures on the
`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.