Additional Dependency

Thanks for this selinux policy for new relic.

In order to run 'make -f /usr/share/selinux/devel/Makefile', as mentioned in the readme, I had to first run 
`yum install selinux-policy-devel` to get the file /usr/share/selinux/devel/Makefile in place. It would probably help others to see this information in the readme.