Added README and gitlab ci configuration

.gitlab-ci.yml

+---
+include:
+  - https://salsa.debian.org/salsa-ci-team/pipeline/raw/master/recipes/debian.yml
+  - https://www.opencode.net/zynthbox/gitlab-ci-tools/raw/main/zynthbox-default-ci.yml
+variables:
+  SOURCE_ARCHIVE: "https://github.com/zynthbox/zynthbox-dependencies/releases/download/RC-5/adlplug-1.0.2.tar.xz"
+  BUILDING_ZYNTHBOX_DEPENDENCY: 1

README.md

+# zynthbox-plugin-adlplug
+
+Build latest zynthbox-plugin-adlplug from source https://github.com/zynthbox/zynthbox-dependencies/releases/download/RC-5/adlplug-1.0.2.tar.xz