Added extension to gitignore
Added extension to gitignore

gusreiber/OSS-plugin-site,gusreiber/jenkins-plugin-site,gusreiber/OSS-plugin-site,scherler/jenkins-plugin-site,scherler/jenkins-plugin-site,gusreiber/jenkins-plugin-site,gusreiber/OSS-plugin-site,scherler/jenkins-plugin-site,gusreiber/jenkins-plugin-site