added a counter example

examples/counter/README.md

@@ -0,0 +1,14 @@
+# Counter
+
+## Description
+
+This directory is a playground for counters in P4. The P4 program includes a
+direct counter and an indirect counter.
+
+### Running the demo
+
+To run the demo:
+- start the switch with `./run_switch.sh`, wait until `READY` is displayed.
+- run the demo script with `./run_demo.sh`. The script will send packets /
+  display the counter values / reset the counters. Look at the script for more
+  details.