Add detailed installation documentation

[willzhang]

What would you like to be added:

1、add docs about how to use coredns docker image，like

docker run -d --name coredns \
  --restart=always \
  -v /etc/coredns/:/etc/coredns/ \
  -p 192.168.72.15:53:53/udp \
  coredns/coredns:1.9.1 -conf /etc/coredns/Corefile

It took me a lot of time to find that i must use -conf /etc/coredns/Corefile , because coredns never report errors .

2、add docs about how to install with go binary，like

wget https://github.com/coredns/coredns/releases/download/v1.9.1/coredns_1.9.1_linux_amd64.tgz
tar -zxvf coredns_1.9.1_linux_amd64.tgz
mv coredns /usr/local/bin

cat >/etc/systemd/system/coredns.service<<EOF
[Unit]
Description=CoreDNS DNS server
Documentation=https://coredns.io
After=network.target
 
[Service]
PermissionsStartOnly=true
LimitNOFILE=1048576
LimitNPROC=512
CapabilityBoundingSet=CAP_NET_BIND_SERVICE
AmbientCapabilities=CAP_NET_BIND_SERVICE
NoNewPrivileges=true
User=coredns
WorkingDirectory=~
ExecStart=/usr/local/bin/coredns -conf=/etc/coredns/Corefile
ExecReload=/bin/kill -SIGUSR1
Restart=on-failure
 
[Install]
WantedBy=multi-user.target
EOF

Why is this needed:

official not say how to install :https://coredns.io/manual/toc/#installation

it not eays for a novice to use coredns as a local dns server.