Add rsync backup guide

.gitignore

+/*.swp

server/rsync-backup.md

+# About this guide
+This guide is for backing file to remote server by using rsync + ssh
+
+# Setup remote server for backup
+
+## Generate SSH key
+
+```
+$ ssh-keygen -f ~/.ssh/id_rsa -q -P ""
+$ cat ~/.ssh/id_rsa.pub
+ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDLVDBIpdpfePg/a6h8au1HTKPPrg8wuTrjdh0QFVPpTI4KHctf6/FGg1NOgM++hrDlbrDVStKn/b3Mu65//tuvY5SG9sR4vrINCSQF++a+YRTGU6Sn4ltKpyj3usHERvBndtFXoDxsYKRCtPfgm1BGTBpoSl2A7lrwnmVSg+u11FOa1xSZ393aaBFDSeX8GlJf1SojWYIAbE25Xe3z5L232vZ5acC2PJkvKctzvUttJCP91gbNe5FSwDolE44diYbNYqEtvq2Jt8x45YzgFSVKf6ffnPwnUDwhtvc2f317TKx9l2Eq4aWqXTOMiPFA5ZRM/CF0IJCqeXG6s+qVfRjB root@cloudads
+```
+
+Copy this key to your clipboard and login to your destination server.
+Place this SSH key into your ~/.ssh/authorized_keys file:
+If your SSH folder does not exist, create it manually:
+
+```
+mkdir ~/.ssh
+chmod 0700 ~/.ssh
+touch ~/.ssh/authorized_keys
+chmod 0644 ~/.ssh/authorized_keys
+```
+
+## Run rsync to copy file
+
+```
+$ rsync -avz --ignore-existing -e "ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null" --progress /root/bigfile.txt ventura@192.168.1.2:/
+```
+
+There are many options of rsync, so please read document to set options suit you.
+
+# Reference links
+https://www.digitalocean.com/community/tutorials/how-to-copy-files-with-rsync-over-ssh
+http://linoxide.com/how-tos/rsync-copy/