add drone CI to project

Squashed commit of the following:

commit 314aa4b989
Author: Julien Fastré <julien.fastre@champs-libres.coop>
Date:   Mon Jul 26 21:29:34 2021 +0200

    restrict ci on tags

commit c39b0d98d9
Author: Julien Fastré <julien.fastre@champs-libres.coop>
Date:   Mon Jul 26 17:59:27 2021 +0200

    add explanation to README about dependencies [ci-skip]

commit b935e1ba68
Author: Julien Fastré <julien.fastre@champs-libres.coop>
Date:   Mon Jul 26 17:55:50 2021 +0200

    clone submodules

commit 8dbc9aa196
Author: Julien Fastré <julien.fastre@champs-libres.coop>
Date:   Mon Jul 26 17:52:23 2021 +0200

    fix drone ci

commit 61fb8a8f66
Author: Julien Fastré <julien.fastre@champs-libres.coop>
Date:   Mon Jul 26 15:34:35 2021 +0200

    first impl for drone

.drone.yml

@@ -0,0 +1,44 @@
+---
+kind: pipeline
+type: docker
+name: default
+
+steps:
+
+- name: submodules
+  image: alpine/git
+  commands:
+  - git submodule init
+  - git submodule update --recursive --remote
+  when:
+    event: tag
+
+- name: pandoc
+  image: pandoc/alpine:2.14
+  commands:
+    - ls
+    - sh build-pandoc.sh latex > user-manual.tex
+  when:
+    event: tag
+
+- name: build-latex
+  image: ghcr.io/xu-cheng/texlive-full:latest
+  commands:
+    - latexmk -pdf -file-line-error -halt-on-error -interaction=nonstopmode -xelatex user-manual.tex
+  when:
+    event: tag
+
+- name: release
+  image: plugins/gitea-release
+  settings:
+    api_key:
+      from_secret: gitea_key
+    base_url: https://gitea.champs-libres.be
+    files:
+      - user-manual.pdf
+    checksum:
+      - sha512
+    title: ${DRONE_COMMIT_REF}
+    prerelease: true
+  when:
+    event: tag