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., `install-dotfiles` and `update-dotfiles`,
   and a `ensure-locales` helper)
- Add README.md with info on the installation and general notes

.config/shell/README.md

@@ -0,0 +1,11 @@
+# Shell-related Configurations
+
+This folder contains files that are sourced by `bash` and `zsh`.
+
+They are not executable.
+The `#!/bin/sh` line documents the dialect for readers and editors;
+    it has no effect, since the files are sourced and never run directly.
+
+[`welcome`](./welcome) and [`logout`](./logout)
+    depend on helper functions defined in [`~/.profile`](../../.profile),
+    so they cannot work standalone.