Start linting rule's doc file

Author: dcyriller

.markdownlint.json

@@ -0,0 +1,3 @@
+{
+  "line-length": false
+}

docs/rules/prettier.md

@@ -1,9 +1,9 @@
-## prettier
+# prettier
 
 Runs [Prettier](https://github.com/prettier/prettier) as a rule. If the code matches prettier output, no issue is raised.
 
 It will use the project's local version of Prettier. Note that `prettier` and `ember-template-lint` are required as peerDependencies of the package that bundles this prettier rule.
 
-### Related Rules
+## Related Rules
 
 - [prettier](https://github.com/prettier/eslint-plugin-prettier) from prettier for eslint

package.json

@@ -36,6 +36,7 @@
     "eslint-plugin-node": "^11.0.0",
     "eslint-plugin-prettier": "^3.1.0",
     "lerna-changelog": "^1.0.0",
+    "markdownlint-cli": "^0.22.0",
     "mocha": "^7.0.1",
     "prettier": "^1.19.0"
   },