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

configs/releng/airootfs/etc/systemd/system/pacman-init.service

+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 [Unit]
 Description=Initializes Pacman keyring
 Wants=haveged.service

configs/releng/airootfs/etc/systemd/system/reflector.service

+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 [Unit]
 Description=pacman mirrorlist update
 Wants=network-online.target

configs/releng/airootfs/etc/systemd/system/systemd-networkd-wait-online.service.d/wait-for-only-one-interface.conf

+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 [Service]
 ExecStart=
 ExecStart=/usr/lib/systemd/systemd-networkd-wait-online --any

configs/releng/airootfs/root/customize_airootfs.sh

 #!/usr/bin/env bash
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
 
 set -e -u
 

configs/releng/airootfs/usr/local/bin/Installation_guide

 #!/bin/sh
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
 
 exec lynx 'https://wiki.archlinux.org/index.php/Installation_guide'

configs/releng/airootfs/usr/local/bin/choose-mirror

 #!/bin/bash
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
 
 get_cmdline() {
     local param

configs/releng/build.sh

 #!/usr/bin/env bash
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
 
 set -e -u
 

configs/releng/efiboot/loader/entries/archiso-x86_64-cd.conf

+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 title   Arch Linux install medium (x86_64, UEFI)
 linux   /EFI/archiso/vmlinuz-linux
 initrd  /EFI/archiso/intel-ucode.img

configs/releng/efiboot/loader/entries/archiso-x86_64-usb.conf

+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 title   Arch Linux install medium (x86_64, UEFI)
 linux   /%INSTALL_DIR%/boot/x86_64/vmlinuz-linux
 initrd  /%INSTALL_DIR%/boot/intel-ucode.img

configs/releng/efiboot/loader/loader.conf

+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 timeout 3
 default archiso-x86_64.conf

configs/releng/isolinux/isolinux.cfg

+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 PATH /%INSTALL_DIR%/boot/syslinux/
 DEFAULT loadconfig
 

configs/releng/mkinitcpio.conf

+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 HOOKS=(base udev memdisk archiso_shutdown archiso archiso_loop_mnt archiso_pxe_common archiso_pxe_nbd archiso_pxe_http archiso_pxe_nfs archiso_kms block filesystems keyboard)
 COMPRESSION="xz"

configs/releng/packages.x86_64

+#
+# SPDX-License-Identifier: GPL-3.0-or-later
 amd-ucode
 arch-install-scripts
 b43-fwcutter

configs/releng/pacman.conf

@@ -2,6 +2,8 @@
 # /etc/pacman.conf
 #
 # See the pacman.conf(5) manpage for option and repository directives
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
 
 #
 # GENERAL OPTIONS

configs/releng/syslinux/archiso.cfg

+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 DEFAULT select
 
 LABEL select

configs/releng/syslinux/archiso_head.cfg

+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 SERIAL 0 38400
 UI boot/syslinux/vesamenu.c32
 MENU TITLE Arch Linux

configs/releng/syslinux/archiso_pxe.cfg

+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 INCLUDE boot/syslinux/archiso_head.cfg
 
 LABEL arch64_nbd

configs/releng/syslinux/archiso_sys.cfg

+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 INCLUDE boot/syslinux/archiso_head.cfg
 
 DEFAULT arch64

configs/releng/syslinux/archiso_tail.cfg

+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 LABEL existing
 TEXT HELP
 Boot an existing operating system.