design doc: add design doc

Add a document to record conclusions from dicsussions in the issues
section.

Design.md

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+# Design Document
+
+This document captures the design decisions made after dicussion in issues. When a design issue is closed, the
+conclusion should be summarized here with a link to the issue.
+
+## Disk Layout
+
+Originally discussed in issue [#18](https://github.com/coreos/fedora-coreos-tracker/issues/18). See also [dustymabe's comment](https://github.com/coreos/fedora-coreos-tracker/issues/18#issuecomment-409668929) summarizing the discussion in the FCOS meeting.
+
+### Summary:
+
+ - FCOS will use a "dd-able" image and ship with a standard parition layout.
+ - The bare metal image and cloud images have the same layout.
+ - Anaconda will not be used for installation.
+ - FCOS should not use the [GPT generator](https://www.freedesktop.org/software/systemd/man/systemd-gpt-auto-generator.html).
+ - LVM should be supported, but not used by default.
+ - Ignition will be used to customize disk layouts.
+
+FCOS should have a fixed partition layout that Ignition can modify on first boot. The installer will be similar to the
+Container Linux installer; the core of it will be dd'ing an image to the disk.
+
+The partition layout is still undecided, but initial proposals look something like:
+
+    Number	Type	Purpose
+    -----------------------------------
+    1	fat32	Boot partition/ESP
+    2	N/A	Bios Boot partition
+    3	?	Root
+    4	?	/var
+
+We also want to support moving the root partition to new locations by recreating the OSTree at the new location. This
+would involve downloading the OSTree repo contents and doing the deploy between the Ignition disks and files stage if
+the root filesystem has changed. This is currently untested.
+
+### Open Questions:
+
+ - What do we do about 4k sector disks? We could make a "hybrid" disk image, but it technically breaks the GPT spec and
+   may not work with poorly implemeted UEFIs.
+ - What is the exact partition layout?
+ - Do we make /etc a ro bind mount?
+ - What filesystem do we use for / and /var? ext4?