Add base configuration

- Follow XDG standard: ~/.config and ~/.local folders
- Set environment variables and define aliases within ~/.config/shell
- Add installation script for easy setup and maintenance
  (i.e., `dotfiles-install` and `dotfiles-update`)
- Add README.md with info on the installation and general notes

.local/state/README.md

@@ -0,0 +1,8 @@
+# State Placeholders
+
+This folder contains empty `.gitkeep`-tracked placeholders
+    for the history and state files
+    set in [`~/.config/shell/env`](../../.config/shell/env)
+    and also other configuration files.
+
+The state files themselves are untracked.