Add gitlab-ci integration and Makefile

Makefile:
Add Makefile, introducing a simple `make lint` target which runs
rstcheck against all .rst files.

.gitlab-ci.yml:
Add GitLab CI integration, with a simple lint target, that calls `make
lint` in an environment with the required dependencies.

.gitlab-ci.yml

+---
+
+default:
+  image: "archlinux:latest"
+
+stages:
+  - lint
+
+rstcheck:
+  stage: lint
+  before_script:
+    - pacman --noconfirm -Syu --needed make rstcheck
+  script:
+    - make lint

Makefile

+DOC_FILES=$(wildcard rfcs/*.rst) $(wildcard *.rst)
+
+all:
+
+check: lint
+
+lint:
+	rstcheck --report warning $(DOC_FILES)