Add description to README.md

README.md

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-# nixpkgs-lpenz
-My overlay/extension of nixpkgs
+[![Build Status](https://travis-ci.com/lpenz/nixpkgs-lpenz.png?branch=master)](https://travis-ci.com/lpenz/nixpkgs-lpenz) 
+
+nixpkgs-lpenz
+=============
+
+## About
+
+This is my overlay/extension of nixpkgs, with a binary cache. It's
+built from [this](https://github.com/lpenz/nixpkgs-lpenz/) github
+repository by using [travis-ci](https://travis-ci.com) and it's
+deployed as a [github pages](https://pages.github.com/).
+
+And for now, it's mostly an example of how to do just that. More
+details [here](http://www.lpenz.org/articles/nixchannel/).
+
+
+## Installation
+
+To use it, add the channel and update it:
+
+~~~[.sh]
+nix-channel --add http://www.lpenz.org/nixpkgs-lpenz/
+nix-channel --update
+~~~
+
+To use the binary cache, you have to add its URL and public key as a
+substituter in `$HOME/.config/nix/nix.conf`:
+
+~~~[.conf]
+substituters = https://cache.nixos.org https://www.lpenz.org/nixpkgs-lpenz/cache
+binary-cache-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= lpenz.org:jYDkTN/2qS96NxFnJpuWkNdVfNaWzaA5pmcg4MyXa2g=
+~~~
+
+You can now test it by installing [execpermfix](http://github.com/lpenz/execpermfix):
+
+~~~[.sh]
+nix-env -iA nixpkgs-lpenz.execpermfix
+~~~