doc: Add note about hackage and stackage update scripts

Author: rvl

doc/maintainer-scripts.md

@@ -0,0 +1,18 @@
+# Updating Hackage and Stackage Nix expressions
+
+The [`hackage.nix`](https://github.com/input-output-hk/hackage.nix)
+and [`stackage.nix`](https://github.com/input-output-hk/stackage.nix)
+repos and corresponding files `hackage-src.json` and
+`stackage-src.json` will be regularly and automatically updated using
+scripts in this repo.
+
+To run the updater scripts manually, use:
+
+    nix-build -A maintainer-scripts.update-hackage -o update-hackage.sh
+    ./update-hackage.sh
+
+    nix-build -A maintainer-scripts.update-stackage -o update-stackage.sh
+    ./update-stackage.sh
+
+The scripts will clone the repo, generate the latest data, then
+attempt to push back to the repo and update the source JSON file.