Mention what an ETL is

intro-stream-processing.html

@@ -301,6 +301,14 @@
 						</ul>
 					</section>
 
+					<section>
+						<ul>
+							<li><strong>E</strong><span class="fragment">xtract</span></li>
+							<li><strong>T</strong><span class="fragment">ransform</span></li>
+							<li><strong>L</strong><span class="fragment">oad</span></li>
+						</ul>
+					</section>
+
 					<section>
 						<p>Batch processing seria o caso de processar os dados de um dia.</p>