add uninstall instructions

docs/uninstall.md

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+# Uninstall IPsec/L2TP VPN
+
+To remove the VPN:
+
+## First step
+```
+service ipsec stop
+service xl2tpd stop
+rm -rf /usr/local/sbin/ipsec /usr/local/libexec/ipsec
+rm -f /etc/init.d/ipsec /lib/systemd/system/ipsec.service
+rm -rf /opt/src/libreswan-*
+```
+
+## Second step
+
+### For Ubuntu/Debian:
+
+`apt-get remove xl2tpd`
+
+### For CentOS/RHEL:
+
+`yum remove xl2tpd`
+
+## Third step
+
+### For Ubuntu/Debian: 
+
+Edit `/etc/iptables.rules` and remove unneeded rules. 
+Your original rules (if any) are backed up as `/etc/iptables.rules.old-date-time`. 
+In addition, edit `/etc/iptables/rules.v4` if the file exists. 
+If using IPv6, also edit `/etc/ip6tables.rules` and `/etc/iptables/rules.v6`.
+
+### For CentOS/RHEL: 
+
+Edit `/etc/sysconfig/iptables` and `/etc/sysconfig/ip6tables`
+
+## Fourth step
+
+Edit `/etc/sysctl.conf` and remove the lines after `# Added by hwdsl2 VPN script`
+
+Edit `/etc/rc.local` and remove the lines after `# Added by hwdsl2 VPN script`, *except exit 0 (if any)*
+
+## Optional
+
+Remove these config files:
+* /etc/ipsec.conf
+* /etc/ipsec.secrets
+* /etc/xl2tpd/xl2tpd.conf
+* /etc/ppp/options.xl2tpd
+* /etc/ppp/chap-secrets
+* /etc/ipsec.d/*
+* /etc/pam.d/pluto
+* /etc/sysconfig/pluto
+
+copy && paste for fast remove:
+
+`sudo rm -f /etc/ipsec.conf /etc/ipsec.secrets /etc/xl2tpd/xl2tpd.conf /etc/ppp/options.xl2tpd /etc/ppp/chap-secrets /etc/ipsec.d/* /etc/pam.d/pluto /etc/sysconfig/pluto`
+
+## When finished
+
+Reboot your server.