Add some README.md text for init triggers.

   3. Any time that property c transitions to value d, while property a already equals b.

Trigger Sequence

----------------

Init uses the following sequence of triggers during early boot. These are the

built-in triggers defined in init.cpp.

   1. `early-init` - The first in the sequence, triggered after cgroups has been configured

      but before ueventd's coldboot is complete.

   2. `init` - Triggered after coldboot is complete.

   3. `charger` - Triggered if `ro.bootmode == "charger"`.

   4. `late-init` - Triggered if `ro.bootmode != "charger"`, or via healthd triggering a boot

      from charging mode.

Remaining triggers are configured in `init.rc` and are not built-in. The default sequence for

these is specified under the "on late-init" event in `init.rc`. Actions internal to `init.rc`

have been omitted.

   1. `early-fs` - Start vold.

   2. `fs` - Vold is up. Mount partitions not marked as first-stage or latemounted.

   3. `post-fs` - Configure anything dependent on early mounts.

   4. `late-fs` - Mount partitions marked as latemounted.

   5. `post-fs-data` - Mount and configure `/data`; set up encryption. `/metadata` is

      reformatted here if it couldn't mount in first-stage init.

   6. `zygote-start` - Start the zygote.

   7. `early-boot` - After zygote has started.

   8. `boot` - After `early-boot` actions have completed.

Commands

--------