title: FHIR : adding a custom extension
body: <p>I would like to add to add a custom extension to my Schedule resource.
  In my app, Schedule have visit motives (reasons). I know there's a list of classified appointments / encounter reasons but I would like to use mine.</p>
  
  <p>I have something like this :</p>
  
  <pre><code>{
    "resourceType":"Schedule",
    "identifier":"logical_id",
    "type":"schedule_speciality",
    "actor":{
      "practioner_id":"identifier",
      "practioner_name":"practioner name"
    },
    "external_id":{
      "extension":[
        {
          "url":"http://api.test.com/fhir/schedule/external_id",
          "valueIdentifier":"external_id"
        }
      ]
    },
    "visit_motives":{
      "extension":[
        {
          "url":"https://api.test.com/fhir/ValueSet/schedule#visit_motives",
          "valueString":"vist_motive1"
        },
        {
          "url":"https://api.test.com/fhir/ValueSet/schedule#visit_motives",
          "valueString":"vist_motive2"
        },
        {
          "url":"https://api.test.com/fhir/ValueSet/schedule#visit_motives",
          "valueString":"vist_motive3"
        }
      ]
    },
    "practice_id":{
      "extension":[
        {
          "url":"https://api.test.com/fhir/schedule/practice_id",
          "valueIdentifier":"practice_id"
        }
      ]
    }
  }
  </code></pre>
  
  <p>I'm not sure about this part :</p>
  
  <pre><code>"visit_motives":{
      "extension":[
        {
          "url":"https://api.test.com/fhir/ValueSet/schedule#visit_motives",
          "valueString":"vist_motive1"
        },
        {
          "url":"https://api.test.com/fhir/ValueSet/schedule#visit_motives",
          "valueString":"vist_motive2"
        },
        {
          "url":"https://api.test.com/fhir/ValueSet/schedule#visit_motives",
          "valueString":"vist_motive3"
        }
      ]
    }
  </code></pre>
  
  <p>Is it correct to add an extension this way ? There are always multiple visit motives for a specific schedule so I have to list them.</p>
  
  <p>I also have seen this kind of things :</p>
  
  <pre><code>"visit_motives": {
            "coding": [
              {
                "system": "https://api.test.com/fhir/ValueSet/schedule#visit_motives",
                "code": "visit_motive1"
              }
            ]
          }
  </code></pre>
  
  <p>Which one is the correct one or am I wrong ?</p>