Docs: verify documentation using markdownlint

This commit bundles a custom version of markdownlint-cli, which includes
markdownlint@0.6.1.

This can be replaced by a regular release of markdownlint-client, once
igorshubovych/markdownlint-cli#21 is merged and
a version is published.

markdownlint is now used for verifying all the markdown files in `docs/`
as a precommit hook and a task that can be used by CI.

.markdownlintrc

@@ -0,0 +1,18 @@
+{
+  "default": false,
+  "MD003": { "style": "atx" },
+  "MD007": { "indent": 4 },
+  "MD012": {
+    "maximum": 2
+  },
+  "MD013": false,
+  "no-trailing-punctuation": {
+    "punctuation": ".,;:!"
+  },
+  "MD033": {
+    "allowed_elements": ["dl", "dt", "dd", "code"]
+  },
+  "first-line-h1": false,
+  "no-hard-tabs": true,
+  "no-trailing-spaces": true
+}

.travis.yml

@@ -25,6 +25,7 @@ before_script:
   # ESLint only supports Node >=4
   - if [ "x$TRAVIS_NODE_VERSION" = "x6" ]; then npm run lint; fi
   # run these on node 6 to have npm 3 with flat dependencies
+  - if [ "x$TRAVIS_NODE_VERSION" = "x6" ]; then npm run lint-markdown; fi
   - if [ "x$TRAVIS_NODE_VERSION" = "x6" ]; then npm run test-headless; fi
   - if [ "x$TRAVIS_NODE_VERSION" = "x6" ]; then npm run test-webworker; fi
   - if [ "x$TRAVIS_NODE_VERSION" = "x6" ] && [ "${TRAVIS_PULL_REQUEST}" = "false" ]; then npm run test-cloud; fi

docs/CONTRIBUTING.md

@@ -39,3 +39,8 @@ bundle exec jekyll serve
 
 After that you can access the site at http://localhost:4000/
 
+## Linting of Markdown
+
+To help keep the documentation syntactically consistent and free of syntax violations, a pre-commit hook using [markdownlint](https://github.com/DavidAnson/markdownlint) verifies Markdown documents.
+
+The CI server uses [markdownlint](https://github.com/DavidAnson/markdownlint) with the same configuration to verify the pull request.