manual: document working on docs

Procfile

@@ -3,3 +3,4 @@ hydra-queue-runner: ./foreman/start-queue-runner.sh
 hydra-evaluator: ./foreman/start-evaluator.sh
 hydra-notify: ./foreman/start-notify.sh
 postgres: ./foreman/start-postgres.sh
+manual: ./foreman/start-manual.sh

doc/manual/src/hacking.md

@@ -44,6 +44,12 @@ if run with high parallelism [due to an issue in
 `Test::PostgreSQL`](https://github.com/TJC/Test-postgresql/issues/40)
 causing database ports to collide.
 
+## Working on the Manual
+
+By default, `foreman start` runs mdbook in "watch" mode. mdbook listens
+at [http://localhost:63332/](http://localhost:63332/) by default, and
+will reload the page every time you save.
+
 ## Building
 
 To build Hydra and its dependencies:

foreman/start-manual.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+mdbook serve \
+  --port 63332 \
+  --dest-dir $(pwd)/.hydra-data/manual \
+  $(pwd)/doc/manual/