Add an FAQ entry to provide workaround for potential bpf pkg installation problem.

Signed-off-by: Yonghong Song <yhs@plumgrid.com>
diff --git a/FAQ.txt b/FAQ.txt
index ddf925a..cc61eff 100644
--- a/FAQ.txt
+++ b/FAQ.txt
@@ -10,6 +10,12 @@
    where libbpfprog.so was installed into your LD_LIBRARY_PATH
    export LD_LIBRARY_PATH=/usr/local/lib:$LD_LIBRARY_PATH
 
+Q: hello_world.py fails with:
+   ImportError: No module named bpf
+A: checkout "sudo make install" output to find out bpf package installation site,
+   add it to the PYTHONPATH env variable before running the program.
+   sudo bash -c 'PYTHONPATH=/usr/lib/python2.7/site-packages python examples/hello_world.py'
+
 Q: hello_world.py still fails with:
    bpf: Operation not permitted
    Exception: Failed to load BPF program hello