docs: add distributor notes

docs/development.md

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 # Development

docs/distributor-notes.md

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+# Distributor Notes
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+1. TOC
+{:toc}
+
+## Kernel Arguments
+
+When Ignition is updating kernel arguments it will call out to a binary (defined in `internal/distro/distro.go` and overridable at build-time via overriding the `github.com/coreos/ignition/v2/internal/distro.kargsCmd` build flag). Ignition expects that the binary accepts `--should-exist` & `--should-not-exist` parameters. Should exist operations should append the argument if missing and should not exist should NOT fail if the argument is not present. The binary should also reboot the system if necessary.
+
+Distributors will also need to create a systemd unit in the initrd, the suggested ordering is after `ignition-fetch.service` and before `ignition-disks.service`.
+
+An example of the binary implementation look at [`examples/ignition-kargs-helper`](https://github.com/coreos/ignition/blob/master/examples/ignition-kargs-helper) & an example service unit is avialable at [`dracut/30ignition/ignition-kargs.service`](https://github.com/coreos/ignition/blob/master/examples/ignition-kargs.service).

docs/supported-platforms.md

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 # Supported Platforms