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

2 files changed, 4 insertions, 0 deletions

diff --git a/configs/releng/airootfs/usr/local/bin/Installation_guide b/configs/releng/airootfs/usr/local/bin/Installation_guide
index cd38645..d790c3e 100755
--- a/configs/releng/airootfs/usr/local/bin/Installation_guide
+++ b/configs/releng/airootfs/usr/local/bin/Installation_guide
@@ -1,3 +1,5 @@
 #!/bin/sh
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
 
 exec lynx 'https://wiki.archlinux.org/index.php/Installation_guide'
diff --git a/configs/releng/airootfs/usr/local/bin/choose-mirror b/configs/releng/airootfs/usr/local/bin/choose-mirror
index 0ae0806..b021945 100755
--- a/configs/releng/airootfs/usr/local/bin/choose-mirror
+++ b/configs/releng/airootfs/usr/local/bin/choose-mirror
@@ -1,4 +1,6 @@
 #!/bin/bash
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
 
 get_cmdline() {
     local param