README.md: add usage instructions

README.md

@@ -16,3 +16,25 @@ This package set has the following policy.
 * The following packages must always build:
   * BiobaseTypes
   * BiobaseXNA
+
+## Updating the Package set
+
+The package set is generated from the `horizon.dhall` using the following command:
+
+```
+nix run 'git+https://gitlab.horizon-haskell.net/haskell/horizon-gen-nix?ref=refs/tags/0.10.0'
+```
+
+If you need to do additional manual overrides to the nix code, such as
+`addPkgconfigDepends`, edit the `configuration.nix` overlay, which is applied
+afterwards.
+
+## Programmmatic Updates
+
+To use `horizon-shell`.
+
+```
+nix run 'git+https://gitlab.horizon-haskell.net/shells/horizon-shell?ref=refs/tags/0.0.7'
+```
+
+See the in-shell help for usage.