Add run-arion-quick + HACKING.md

HACKING.md

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+
+# Hacking on the modules
+
+## Easiest option
+
+The module system does not distinguish between modules and configurations.
+This mean you can prototype any feature by factoring out functionality in a real-world project.
+
+## Changing the built-in modules
+
+If your change is not just an addition or if it's better implemented by refactoring, you'll want to fork and edit arion sources directly.
+
+For a fast iteration cycle (but possibly outdated arion command logic):
+
+    ~/src/arion/run-arion-quick up -d
+
+To update the arion command logic on the next run
+
+    rm ~/src/arion/result-run-arion-quick
+
+
+# Hacking on the arion command
+
+The arion command is written in Haskell. Anyone can make small changes to the code.
+Experience with Haskell tooling is not required. You can use the nixified scripts in the root of the repo for common tasks.
+ - `build` or `live-check` for typechecking
+ - `live-unit-tests` (only the test suite is "live" though)
+ - `repl` for a Haskell REPL
+ - `run-arion` to run an incrementally built arion
+ - `run-arion-via-nix` to run a nix-built arion
+ - ~~`run-arion-quick`~~ *not for command hacking;* use stale command. See previous section.

run-arion-quick

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+#!/usr/bin/env bash
+
+projectRoot="$(dirname ${BASH_SOURCE[0]})"
+resultLink="$projectRoot/result-run-arion-quick"
+
+[[ -e "$resultLink" ]] || {
+  echo 1>&2 "You don't have a prebuilt arion yet; building it."
+  nix-build "$projectRoot" -A arion --out-link "$resultLink"
+}
+
+echo 1>&2 "Note that you will need to rm '$resultLink' to rebuild the arion executable when needed."
+
+export arion_compose_datadir="$projectRoot/src"
+
+exec "$resultLink/bin/arion" "$@"