In a RAML file (used for creating API docs), you can add arbitrary content outside of the endpoints by using the documentation key, an array where each object has a title property and Markdown-formatted content property.

Example from the spec:

#%RAML 1.0
title
: ZEncoder API
baseUri
: https://app.zencoder.com/api
documentation
:
 - title
: Home
   content
: |
    Welcome to the _Zencoder API_ Documentation. The _Zencoder API_
     allows you to connect your application to our encoding service
     and encode videos without going through the web  interface. You
     may also benefit from one of our
     [integration libraries](https://app.zencoder.com/docs/faq/basics/libraries)
     for different languages.

 - title
: Legal
   content
: !include docs/legal.markdown
Final thoughts

Not sure why it took me ages to find this, it's right there in the first sections of the spec.