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

Signed-off-by: Yonghong Song <yhs@plumgrid.com>

FAQ.txt

@@ -10,6 +10,12 @@ A: make sure to 'make install' and add the directory
    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

README.md

@@ -73,8 +73,6 @@ sudo python examples/hello_world.py
 ```
 
 Change `CMAKE_PREFIX_PATH` if llvm is installed elsewhere.
-Change CMAKE_INSTALL_PREFIX to /usr/local if your python package installation
-is under /usr/local install /usr.
 
 ### Cleaning up