Add .gitlab-ci.yml with GitLab Pages support

.gitlab-ci.yml

+# All available Hugo versions are listed here: https://gitlab.com/pages/hugo/container_registry
+image: registry.gitlab.com/pages/hugo:latest
+
+test:
+  script:
+    - hugo
+  except:
+    - master
+
+pages:
+  script:
+    - hugo
+  artifacts:
+    paths:
+      - public
+  only:
+    - master
+  tags:
+    - secure