Merge pull request #839 from matthiasdebernardini/patch-1

Update yaml-options.md with clear instructions for adding packages that aren't part of Nix packages

docs/reference/yaml-options.md

@@ -53,3 +53,20 @@ imports:
 !!! note "Added in 1.0"
 
     - relative file support in imports: `./mymodule.nix`
+
+### What if a package is out of date?
+    - Open [nixpkgs repo](https://github.com/NixOS/nixpkgs) and press `t` to search for your package.
+    -  Try to update/change the package using [the nixpkgs contributing guide](https://nixos.org/manual/nixpkgs/stable/#chap-quick-start), optionally contacting the maintainer for help if you get stuck.
+    - Make a PR and remember the branch name.
+    - Add it to your devenv.yaml like so
+
+Using the nixpkgs input in form of 'github:$GH_USERNAME/nixpkgs/master', edit `devenv.yaml`:
+
+
+```yaml
+inputs:
+  nixpkgs:
+    url: 'github:$GH_USERNAME/nixpkgs/MYBRANCH'
+```
+
+