doc: expand minimal readme to (minimal +1)

Author: blaggacao

README.md

@@ -6,4 +6,18 @@
 
 ---
 
-<sub>It is forbidden to modify this readme &rarr; read the comments in <a href="flake.nix"><code>flake.nix</code></a>!</sub>
+_A collection of common devshell modules specified via [`numtide/devshell`][devshell]._
+
+## Why?
+
+- Avoid repetition and reuse common best practices.
+- Provide a collection of components to "kill the toil" in the devshell in different scenarios.
+- Integrate DevShell concerns with the wider config management in `nix`: cross-reference variables.
+
+## Usage
+
+This is a prototype: consult usage with your Resident SRE.
+
+<sub>For more details: &rarr; read the comments in <a href="flake.nix"><code>flake.nix</code></a>!</sub>
+
+[devshell]: https://github.com/numtide/devshell