Add readme

In order to document the contents of the folder.
How to install and run daily tasks like updating.

Also might increase in future to include further info.

readme.rst

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+nixpkgs
+=======
+
+My personal ``~/.config/nixpkgs`` folder, for sharing, inspiration and retrieving feedback.
+
+.. contents:: Table of Contents
+
+Installation
+------------
+
+Install (clone) into ``~/.config/nixpkgs``.
+
+Usage
+-----
+
+Install packages as usual.
+
+``my-packages``
+---------------
+
+The special ``my-packages`` includes a derivation which holds a set of packages.
+The idea: Provide a configuration instead of some state.
+It can be installed via ``nix-env -i my-packages``
+which also "updates" state accordingly to the current defined set.
+
+Update
+------
+
+The following will update the whole system and clean things up::
+
+    nix-channel --update \
+      && nix-env -iA nixpkgs.nix nixpkgs.cacert \
+      && nix-env --upgrade \
+      && nix-env --delete-generations +5 \
+      && nix-store --gc
+
+This will update the channel (fetch state of nixpkgs).
+It then will update nix itself.
+Then upgrade all installed derivations (packages).
+Delete old generations, except last 5.
+And run garbage collection.
+
+Disclaimer
+----------
+
+This is my personal setup for my local laptop running Ubuntu.
+I share this for free so everyone can have a look and inspiration or provide feedback to me.
+
+This is not intended to be used by anyone else just by copy and pasting or cloning.