Add markdownlint test on Travis CI

Author: PeterDaveHello

.travis.yml

@@ -38,6 +38,13 @@ jobs:
       - npm i -g echint
     script:
       - echint
+  - stage: Static tests
+    env:
+      - task=markdownlint
+    services:
+      - docker
+    script:
+      - docker run -it --rm -v "$(pwd)":/md peterdavehello/markdownlint markdownlint '**/*.md'
   - stage: Static tests
     env:
       - task=shfmt