Add ARH file format description

.gitlab-ci.yml

+image: blang/latex
+
+build:
+  script:
+    - latexmk -pdf file_format.tex
+  artifacts:
+    paths:
+      - "*.pdf"

README.md

+# ARH3 file-format description
+
+Download the latests [Format Specification](https://gitlab.mn.tu-dresden.de/spraetor/arh3/-/jobs/artifacts/master/raw/file_format.pdf?job=build)