Update README.md to include cache chapter

Explains how to configure your project/local system in order to use
Cachix/the local cache.

Author: jaschutte

README.md

@@ -23,8 +23,41 @@ You can also look through Adam Walker's excellent list of [prebuilt Clash circui
 # Nix
 This project exposes several Nix flake outputs. Most notably the `clash-cores` package, exposed individually or as an overlay (which you need to apply on top of clash-compiler).
 
+## Usage
+
 Contributing to `clash-cores` can be done via the developer shell exposed by the Nix flake. Open a terminal and type: `nix develop`. Optionally a specific GHC version can be selected as well as a `-minimal` or `-full` version. The `-minimal` shell does **NOT** contain the Haskell Language Server, whilst the `-full` shell does. When using the developer shell, do not forget to remove the `cabal.project` file. This file contains a package source and Cabal prioritizes local package sources over Nix sources. Removing the `cabal.project` file should work fine when developing with Nix.
 
+Compiling Clash and all dependencies related to it may take a long time. To remidy long compilation times, you can make use of the publically available cache on Cachix:
+
+### !! ADD CACHIX HELP !! ###
+
+Cachix contains all commits on the main branch of `clash-cores` since the cache has been setup.
+
+### QBayLogic
+
+Employees working at QBayLogic also have access to a local binary cache, which contains every version of `clash-cores` (also those outside of the main branch). To access the local binary cache you can either add it to your `nix.conf` or on a project basis in the `flake.nix`.
+
+`nix.conf`
+```conf
+extra-substituters = http://localhost:9200/public
+extra-trusted-public-keys = public:PGGlJMx1gGmU069blMqve8tS1ndzBuriUAwGBHGOo4g=
+```
+
+`flake.nix`
+```nix
+{
+  nixConfig = {
+    extra-substituters = [ "http://localhost:9200/public" ];
+    extra-trusted-public-keys = [ "public:PGGlJMx1gGmU069blMqve8tS1ndzBuriUAwGBHGOo4g=" ];
+  };
+  description = ...;
+  inputs = ...;
+  outputs = ...;
+}
+```
+
+## As a dependency
+
 You can also add this project as a Nix-dependency. This project depends on `clash-compiler`, the recommended way to add this to your project as a Nix dependency is as follows:
 
 First add `clash-compiler` and `clash-cores` to your flake inputs: