bson/src/main/org/bson/Document.java
public Document append(final String key, final Object value) {
        documentAsMap.put(key, value);
        return this;
    }
public Document append(final String key, final Object value) {
        documentAsMap.put(key, value);
        return this;
    }
Put the given key/value pair into this Document and return this.  Useful for chaining puts in a single expression, e.g.
<pre>
doc.append("a", 1).append("b", 2)}
</pre>
@param key   key
@param value value
@return this