docs: Add some README, API and config docs

docs/api.md

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+
+# bootcfg API
+
+## iPXE Script
+
+Serves a static iPXE boot script which gathers client machine attributes and chain loads to the iPXE boot config endpoint. Configure your iPXE server or DHCP options to boot from this script (e.g. set `dhcp-boot:dhcp-boot=tag:ipxe,http://provisioner.example.com/ipxe/boot.ipxe` if using `dnsmasq`).
+
+    GET http://bootcfg.example.com/boot.ipxe
+
+**Response**
+
+    #!ipxe
+    chain config?uuid=${uuid}
+
+## iXPE Boot Config
+
+Renders an iPXE boot config script based on hardware attribute query parameters and registered boot configs. Hardware attributes are matched in priority order: UUID, MAC address, default (if set).
+
+    GET http://bootcfg.example.com/ipxe
+
+**Query Parameters**
+
+| Name | Type   | Description   |
+|------|--------|---------------|
+| uuid | string | Hardware UUID |
+| mac  | string | MAC address   |
+
+**Response**
+
+    #!ipxe
+    kernel kernel-for-machine.vmlinuz cmd-line-options
+    initrd initrd-for-machine
+    boot
+
+`kernel-for-machine`, `cmd-line-options`, and `initrf-for-machine` are populated from a boot config.
+
+## Pixiecore Boot Config
+
+Renders a Pixiecore boot config as JSON to implement the Pixiecore API [spec](https://github.com/danderson/pixiecore/blob/master/README.api.md) for Pixiecore setups. Currently, Pixiecore only provides MAC addresses for mapping to boot configs.
+
+    GET http://bootcfg.example.com/pixiecore/v1/boot/:MAC
+
+**URL Parameters**
+
+| Name | Type   | Description |
+|------|--------|-------------|
+| mac  | string | MAC address |
+
+**Response**
+
+    {
+      "kernel": "\/images\/coreos_production_pxe.vmlinuz",
+      "initrd": [
+        "\/images\/coreos_production_pxe_image.cpio.gz"
+      ],
+      "cmdline": {
+        "coreos.autologin": ""
+      }
+    }
+
+## Cloud Configs
+
+Serves cloud configs based on hardware attribute query parameters and registered cloud configs. Hardware attributes are matched in priority order: UUID, MAC address, default (if set).
+
+    GET http://bootcfg.example.com/cloud
+
+**Query Parameters**
+
+| Name | Type   | Description   |
+|------|--------|---------------|
+| uuid | string | Hardware UUID |
+| mac  | string | MAC address   |
+
+**Response**
+
+    #cloud-config
+    coreos:
+      units:
+        - name: etcd2.service
+          command: start
+        - name: fleet.service
+          command: start
+
+## Image Assets
+
+If you need to host kernel and initrd images within your network, bootcfg server's `/images/` route serves free-form static assets. Set the `-images-path` when starting the bootcfg server. Here is an example images directory layout:
+
+    images/
+    └── coreos
+        └── 835.9.0
+            ├── coreos_production_pxe.vmlinuz
+            └── coreos_production_pxe_image.cpio.gz
+        └── 877.1.0
+            ├── coreos_production_pxe.vmlinuz
+            └── coreos_production_pxe_image.cpio.gz
+

docs/config.md

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+
+# Config: Flags and Variables
+
+`bootcfg` arguments can be provided as flags or as environment variables.
+
+| flag | varaible | example |
+|------|----------|---------|
+| -address | BOOTCFG_ADDRESS | 0.0.0.0:8080 |
+| -data-path | BOOTCFG_DATA_PATH | ./data |
+| -images-path | BOOTCFG_IMAGES_PATH | ./images, ./static |
+| -log-level | BOOTCFG_LOG_LEVEL | critical, error, warning, notice, info, debug |
+
+## Examples
+
+Binary
+
+    ./run -address=0.0.0.0:8080 -data-path=./data -images-path=./images -log-level=info
+
+Container
+
+    docker run -p 8080:8080 --name=bootcfg --rm -v $PWD/data:/data -v $PWD/images:/images coreos/bootcfg:latest -address=0.0.0.0:8080 -data-path=./data -images-path=./images
+