Document npm install [skip ci]

Author: adamtheturtle

CONTRIBUTING.md

@@ -6,12 +6,18 @@ Travis CI is the canonical source truth.
 
 ## Install Contribution Dependencies
 
-Install dependencies in a virtual environment.
+Install Python dependencies in a virtual environment.
 
 ```sh
 pip install --editable .[dev]
 ```
 
+Insall `npm` dependencies for linting Markdown files:
+
+```
+npm install -g markdownlint-cli
+```
+
 Spell checking requires `enchant`.
 This can be installed on macOS, for example, with [Homebrew](http://brew.sh):