Update README.md

Update README.md

update before installation

check file before copying

crl.pem generating fix

openvpn check and deluser (revoke); crl-verify option added; adduser bugfix

uninstall added; bugfixes; adduser dir changed

iptables fix

default reject rules removing

openssl.cnf workaround fix

platform detection fix

CentOS support; copying (adduser) bugfixes; adduser continuation fix

checkserver fix (for using systemctl); install.sh small refactoring; openvpn some dists bugfix (systemctl with full config name)

misspell; continue bugfix; mkdir -p option

Update options.xl2tpd.dist