doc: document supported types of forking/non-forking daemon's

Issue #223

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/service.md

@@ -1,6 +1,50 @@
 Finit Services
 ==============
 
+Starting & Monitoring
+---------------------
+
+Finit can start and monitor the following types of daemons:
+
+* Forks to background, creates a PID file
+* Runs in foreground:
+  - creates a PID file
+  - does not create a PID file -- Finit can create it for you (optional)
+
+Finit can *not* start and monitor a daemon that:
+
+* Forks to background and does *not* create a PID file
+
+|   | Forking | Creates PID File | Finit can create PID File |
+|---|---------|------------------|---------------------------|
+| ✔ | Yes     | Yes              | N/A                       |
+| ✔ | No      | Yes              | N/A                       |
+| ✔ | No      | No               | Yes                       |
+| ✘ | Yes     | No               | No                        |
+
+> **Note:** PID files is one mechanism used to assert conditions to
+> synchronize the start and stop of other, dependent, services.
+
+### Forks to Background w/ PID File
+
+    service pid:!/run/serv.pid serv       -- Forking service
+
+### Runs in Foreground w/ PID File
+
+    service                    serv -n -p -- Foreground service w/ PID file
+
+### Runs in Foreground w/o PID File
+
+Same as previous, but we tell Finit to create the PID file, because we
+need it to synchronize start/stop of a dependent service.
+
+    service pid:/run/serv.pid  serv -n    -- Foreground service w/o PID file
+
+### Runs in Foreground w/ Custom PID File
+
+    service pid:/run/servy.pid serv -n -p -P /run/servy.pid -- Foreground service w/ custom PID file
+
+
 ![The service state machine](svc-machine.png "The service state machine")
 
 State Machine