title: REST API routes URI - fetch by id or by a custom field
body: <p>I'm building a REST API and I would like to know what are the URI best practices. I have "users" entities, that have a unique email and ID. Then I can fetch a user using this routes:    </p>
  
  <pre><code> GET /users/{id}
   GET /users/{email}
  </code></pre>
  
  <p>Should I expose 2 differents routes in order to fetch either by ID or EMAIL?  </p>
  
  <p>I looked at Github API, and they use Field identifier for some of the ressources (/:owner/:repo) and ID number for others.</p>