Add optional kernel flags for running networking examples.

All of these flags are used in networking examples. Like tc policing,
action, dummy interface, vxlan support These flags have
already been enabled in some Linux distributions, like Ubuntu.
But they are disabled by default on vanilla kernels. It would be nice to
mention them in INSTALL.md to make bcc easiser to use.
Signed-off-by: Zhiyi Sun <zhiyisun@gmail.com>

INSTALL.md

@@ -30,6 +30,16 @@ CONFIG_HAVE_BPF_JIT=y
 CONFIG_BPF_EVENTS=y
 ```
 
+There are a few optional kernel flags needed for running bcc networking examples on vanilla kernel:
+
+```
+CONFIG_NET_SCH_SFQ=m
+CONFIG_NET_ACT_POLICE=m
+CONFIG_NET_ACT_GACT=m
+CONFIG_DUMMY=m
+CONFIG_VXLAN=m
+```
+
 Kernel compile flags can usually be checked by looking at `/proc/config.gz` or
 `/boot/config-<kernel-version>`.