Merge pull request #73 from DamienCassou/add-binary-cache-doc

Add FAQ section "How do I add a binary cache?" to README

README.md

@@ -78,19 +78,21 @@ To install Nix from any commit, go to [the corresponding installer_test action](
 ### How do I print nixpkgs version I have configured?
 
 
-```nix-instantiate --eval -E '(import <nixpkgs> {}).lib.version'```
+```yaml
+- name: Print nixpkgs version
+  run: nix-instantiate --eval -E '(import <nixpkgs> {}).lib.version'
+```
 
 ### How can I run NixOS tests?
 
 With the following inputs:
 
 ```yaml
-    - uses: cachix/install-nix-action@vXX
+- uses: cachix/install-nix-action@vXX
   with:
     extra_nix_config: "system-features = nixos-test benchmark big-parallel kvm"
 ```
 
-
 [Note that there's no hardware acceleration on GitHub Actions.](https://github.com/actions/virtual-environments/issues/183#issuecomment-610723516).
 
 ### How can I install packages via nix-env from the specified `nix_path`?
@@ -99,6 +101,31 @@ With the following inputs:
 nix-env -i mypackage -f '<nixpkgs>'
 ```
 
+### How do I add a binary cache?
+
+If the binary cache you want to add is hosted on [Cachix](https://cachix.org/) and you are
+using [cachix-action](https://github.com/cachix/cachix-action), you
+should use their `extraPullNames` input like this:
+
+```yaml
+- uses: cachix/cachix-action@vXX
+   with:
+     name: mycache
+     authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+     extraPullNames: nix-community
+```
+
+Otherwise, you can add any binary cache to nix.conf using
+install-nix-action's own `extra_nix_config` input:
+
+```yaml
+- uses: cachix/install-nix-action@vXX
+  with:
+    extra_nix_config: |
+      trusted-public-keys = hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
+      substituters = https://hydra.iohk.io https://cache.nixos.org/
+```
+
 ## Hacking
 
 Install the dependencies