Add missing @since tag

Change-Id: I80cfc3e81d5cebf810f36ddf2760aee955c6f50a
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/api/Git.java

@@ -406,6 +406,7 @@ public class Git {
 	 * Returns a command to create an archive from a tree
 	 *
 	 * @return a {@link ArchiveCommand}
+	 * @since 3.1
 	 */
 	public ArchiveCommand archive() {
 		return new ArchiveCommand(repo);