Update buildpack documentation

Author: zfields

docs/img/buildpack-dependency-chart.png

@@ -11,37 +11,43 @@ There are four main repositories involved in generating the environment.
 - [`buildpack-hal`](https://github.com/particle-iot/buildpack-hal/): Contains the `Dockerfile` that specifies the contents of the environment, which is consumed by CI, CD, WebIDE and the Workbench.
 - [`device-os`](https://github.com/particle-iot/device-os/): Contains the `.workbench/manifest.json` file, which specifies the necessary components.
 - [`firmware-buildpack-builder`](https://github.com/particle-iot/firmware-buildpack-builder/): Contains the Dockerfile fragments, which are used during CI to validate builds and create the images consumed by the WebIDE compiler.
-- [`workbench`](https://github.com/particle-iot/workbench/): Contains the dependency installer script that consumes the manifest file from the `device-os` repository.
+- [`workbench`](https://github.com/particle-iot/workbench/)*: Contains the dependency installer script that consumes the manifest file from the `device-os` repository.
 
 Relationship Mapping
 --------------------
 
+!["Buildpack Dependency Chart"](./img/buildpack-dependency-chart.png "Circular Dependency")
+
 Examples
 --------
 
 ### Upgrading the `gcc-arm` compiler
 
-#### Create the dependency bundle
+1. Create the dependency bundle
+
+   1. Download the dependency for your platform
+   1. [Create a bundle](https://github.com/particle-iot/workbench#development)
+   1. Generate a SHA-256 hash for the bundle
+   1. Upload the bundle to Amazon S3
+
+1. Update `device-os`
 
-1. Download the dependency for your platform
-1. [Create a bundle](https://github.com/particle-iot/workbench#development)
-1. Generate a SHA-256 hash for the bundle
-1. Upload the bundle to Amazon S3
+   1. Update `<particle-iot/device-os>/.workbench/manifest.json` to reflect the new dependency
+   1. Update the `BUILDPACK_BUILDER_VERSION_TAG` variable to tell Travis CI to use the new `branch` or `tag` from `<particle-iot/device-os>/scripts/fetch-buildpack`
+   1. Push a new branch to the `particle-iot/device-os` repository
 
-#### Update `device-os`
+   > _**NOTE:** Travis CI will fail, because the new `branch` or `tag` in `firmware_buildpack_builder` can't exist due to the circular dependency (depicted above)._
 
-1. Update `<particle-iot/device-os>/.workbench/manifest.json` to reflect the new dependency
-1. Update the `BUILDPACK_FILES` variable to target new `branch` or `tag` in `<particle-iot/device-os>/scripts/fetch-buildpack`
-1. Create a PR against `particle-iot/device-os`
+1. Update `buildpack-hal`
 
-#### Update `buildpack-hal`
+   1. Update the `GITHUB_BRANCH` variable in the `build-and-push` script, to identify which `device-os` branch will provide the updated version of the manifest file
+   1. Update the `COPY` command in the `Dockerfile` to copy the new dependency from the `worker` stage to the `buildpack-hal` stage
+   1. Tag the commit with a version to be used. (i.e. `0.1.0`)
+   1. Push a new branch to the `particle-iot/buildpack-hal` repository
 
-1. Update the `GITHUB_BRANCH` in the `build-and-push` script, to identify which `device-os` branch will provide the updated version of the manifest file
-1. Update the `COPY` command in the `Dockerfile` to copy the new dependency from the `worker` stage to the `buildpack-hal` stage
-1. Tag the commit with a version to be used. (i.e. `0.1.0`)
-1. Create a PR against `particle-iot/buildpack-hal`
+1. Update `firmware-buildpack-builder`
 
-#### Update `firmware-buildpack-builder`
+   1. Update `BUILDPACK_VERSION` in `<particle-iot/device-os>/scripts/constants` to reflect the name of the tag provided to `buildpack-hal`, which was used to tag the resulting Docker image
+   1. Push a new branch to the `firmware-buildpack-builder` repository
 
-1. Update `BUILDPACK_VERSION` in `<particle-iot/device-os>/scripts/constants` to reflect the name of the tag provided to `buildpack-hal`
-1. Create a PR against `firmware-buildpack-builder`
+1. Restart the original `device-os` Travis CI job _(above)_