Add license and basic documentation

LICENSE:
Add GPL-3.0 license.

{{archiso,configs}/*,.editorconfig,.gitlab-ci.yml}:
Add SPDX license identifier.

Makefile:
Add SPDX license identifier.
Install the `run_archiso.sh` script as global executable `run_archiso`.
Use -D and -t flags to install to install files more generically (without a previous call to install the directory).

README.rst:
Add README outlining the project's scope, how to build images from the profiles and how to test.

AUTHORS.rst:
Add list of all direct contributors to the repository.

CONTRIBUTING.rst:
Add basic contribution guidelines, explaining the linter and the license in use.

Closes #7
Closes #3