adde source_routing exercise

SIGCOMM_2015/source_routing/p4src/source_routing.p4

@@ -0,0 +1,28 @@
+/*
+Copyright 2013-present Barefoot Networks, Inc. 
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+parser start {
+    // TODO
+    return ingress;
+}
+
+control ingress {
+    // TODO
+}
+
+control egress {
+    // leave empty
+}