doc: Add service state machine documentation

doc/service.md

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+# Services
+
+## State Machine
+
+![A service's state machine](svc-machine.png "A service's state machine")
+
+A service is bound to a state machine that is in one of six (6)
+states. For `run`s and `task`s there is an additional end state called
+`DONE`, not shown in the diagram for brevity. All services start in
+the `HALTED` state.
+
+The current state depends on the two following conditions:
+
+* `E`: Service enabled. In order for `E` to be satisfied, the service
+  must be allowed to run in the current runlevel and must not be
+  blocked. A service may be blocked for several reasons:
+
+  - The user has manually stopped the service using `initctl stop
+    JOB`.
+
+  - The binary repeatedly dies after being started. I.e. keeps
+    crashing.
+
+  - The binary is missing in the filesystem.
+
+* `C`: Service conditions are satisfied.