Build kernel in another directory and README.md update for fuel-plugin

Build kernel in another directory and README.md update for fuel-plugin

1. Since some files generated during kernel building are only root
   writeable, this will cause "permission denied" issue in "make clean"
   in fuel@opnfv iso building which uses normal user priviledge.
   Building kernel in another directory than the directory mapped into
   Docker container can resolve the issue.
2. Update and typing error correction for README.md

Upstream status - NA

Change-Id: Icdd1a12d38f90619dd91588cb9e0f5b9bdfe95d1
Signed-off-by: davidjchou <david.j.chou@intel.com>