Add installation script

- ~/.config/shell/init_dofiles.sh "installs" the provided dotfiles
  when sourced in an interactive shell
- see the README.md for instructions

README.md

@@ -1,3 +1,26 @@
 # Dotfiles
 
 This repository contains useful (config) files that I use on my machines.
+
+
+## Initialization
+
+On a freshly set up machine, run:
+
+```bash
+curl https://gitlab.webartifex.biz/alexander/dotfiles/-/raw/main/.config/shell/init_dotfiles.sh \
+     > /tmp/init_dotfiles.sh \
+  && source /tmp/init_dotfiles.sh
+```
+
+This gives you a local copy of the latest version of this repository
+    (located in `~/.dotfiles`)
+    and initializes all the dotfiles provided here on the system.
+Further, `zsh` is set up
+    with [`oh-my-zsh`](https://ohmyz.sh/) and [`zplug`](https://github.com/zplug/zplug)
+    if it is installed.
+
+**Note**: Log out and in again so that `bash` and `zsh` run as *login* shells.
+Otherwise, `~/.profile` is probably *not* sourced.
+
+**Important**: Don't forget to back up your current dotfiles!