update doc on config security

* current                                                             :entry:

** 

** 2019-11-23 when default .ssh dir is in %programdata%\mbackup\.ssh,

there is a security concern.

when admin (one user) config mbackup target and ssh-key.

other users may not want their files to be backed up to admin configured server.

In current situation, user's files will be backed up without any user confirmation.

- should I use global backup with global configuration or should mbackup always

  use per user backup config?

  For system files, local-list.txt and local-exclude.txt, use global configuration.

  only backup current users's home dir if user have accepted to do so.

  create a GUI to let user enable mbackup for current user.

  implementation is easy. just add a reg key or create an empty file for current user.

  So, mbackup is always global. User can choose whether they want their files to be backed up.

- is it a security issue that config file can be modified by non-admin?

  non-admin can change target dir to a dir he/she has access to, then read other user's files?

  mbackup process won't be able to read other user's files.

  but when admin user login, and runs mbackup. the non-admin user will now be able to

  read other user's files.

  Security requires more thinking. Current setup is for trusted PC, such as single user PC

  or family PC. It's not ready for multi-user PC, workstation or Windows server.

** 2019-11-23 add an icon for desktop shortcut.

use recognizable backup icon, with some color.

** 2019-11-18 add dockerfile to create rsync mingw64 distribution,