title: pouchdb : add key/value to document
body: <p>I am using pouchDB, but I am quite new to it.
  I would like to add a key/value to a document in my users database.
  Here's my attempt :</p>
  
  <pre><code>this.addToExistingUser = function(docId,key,value) {
      usersDatabase.get(docId).then(function(doc) {
        return db.put({
          _id: docId,
          _rev: doc._rev,
          key: value
        });
      }).then(function(response) {
        alert("done eee");
      }).catch(function (err) {
        console.log("error from addToExistingUser:");
        console.log(JSON.stringify(err));
      });
  }
  </code></pre>
  
  <p>where docId is the _id of the document I am targeting.
  By the way, can I use the email field to point to the document I am targeting  ?</p>
  
  <p>I get this error :</p>
  
  <pre><code>{"status":404,"name":"not_found","message":"missing","error":true,"reason":"missing"}
  </code></pre>
  
  <p>Thanks</p>