Troubleshooting tips regarding SELinux

SELinux may prevent WebPG and even GPG from working correctly, according to its default policies. This commit brings instructions for end users to fix it so that they can enjoy the best out of WebPG.
Merge pull request #1 from mribeirodantas/troubleshooting

Merge pull request from @mribeirodantas; "Troubleshooting tips regarding SELinux"

Thanks! I will update the guide subsystem to include this.