Add DEPLOYING.md

Document how to spin up virtual machine deployments using the
tools/deploy-* scripts.

DEPLOYING.md

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+# Deploying osbuild-composer
+
+*osbuild-composer* has currently has to be deployed in a virtual machine. The
+[tools](./tools) subdirectory contains various scripts (those starting with
+`deploy-`) to deploy it into cloud-init-enabled environemnts. These scripts all
+take the form:
+
+    $ ./tools/deploy-<target> <config> <userdata>
+
+`<config>` depends on the target (see below). `<userdata>` is either a
+cloud-init [cloud-config file][cloud-config], or a directory containing
+this configuration, as documented by [./tools/gen-user-data][gen-user-data].
+
+[gen-user-data]: ./tools/gen-user-data
+[cloud-config]: https://cloudinit.readthedocs.io/en/latest/topics/format.html#cloud-config-data
+
+
+## Target: QEMU
+
+`tools/deploy-qemu` takes as `<config>` the path to a qcow2 image and starts a
+ephemeral virtual machine using qemu. The qcow2 file is not changed and all
+changes to the virtual machine are lost after stopping qemu.
+
+Two ports are forwarded to the host via qemu's [user networking][qemu-network]:
+22 → 2222 and 443 → 4430.
+
+See [HACKING.md][./HACKING.md] for how to use this target for running
+integration tests locally.
+
+[qemu-network]: https://wiki.qemu.org/Documentation/Networking#User_Networking_.28SLIRP.29
+
+
+## Target: OpenStack
+
+`tools/deploy-openstack` uses the `openstack` tool (from `python3-openstack`)
+to deploy a machine in an OpenStack cluster. It expects that an [OpenStack RC
+file][openstackrc] was sourced into the running shell:
+
+    . openstackrc.sh
+
+`<config>` has to be a JSON-file containing configuration about what kind of
+machine to create. For example:
+
+```json
+{
+  "name": "composer-instance",
+  "image": "fedora-32-x86_64",
+  "flavor": "m1.small",
+  "network": "my-network-id"
+}
+```
+
+[openstackrc]: https://docs.openstack.org/newton/admin-guide/common/cli-set-environment-variables-using-openstack-rc.html