Add migration guide for Buildpack 0.10

Signed-off-by: Natalie Arellano <[email protected]>

Author: Natalie Arellano

content/docs/reference/spec/migration/buildpack-api-0.9-0.10.md

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+title="Buildpack API 0.9 -> 0.10"
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+
+<!--more-->
+
+This guide is most relevant to buildpack authors.
+
+See the [spec release](https://github.com/buildpacks/spec/releases/tag/buildpack%2Fv0.10) for Buildpack API 0.10 for the full list of changes and further details.
+
+### Stacks are deprecated
+
+In Buildpack 0.10, the concepts of stacks and mixins are removed
+in favor of existing constructs in the container image ecosystem such as operating system name, operating system distribution, and architecture.
+
+#### Before build
+
+`builder.toml` contains a new table for buildpacks to express the os/arch combinations that they are compatible with:
+
+```toml
+[[targets]]
+os = "<OS name>"
+arch = "<architecture>"
+variant = "<architecture variant>"
+[[targets.distros]]
+name = "<OS distribution name>"
+version = "<OS distribution version>"
+```
+
+All fields are optional and any missing field is assumed to "match any".
+
+This information will be used by the lifecycle to skip running detect on any buildpack that is not compatible with the current os/arch.
+
+Note that the `[[stacks]]` table is still supported and buildpack authors are encouraged to continue to provide this information for the time being
+in order to maintain compatibility with older platforms.
+
+#### During build
+
+The lifecycle will provide the following environment variables during `detect` and `build` to describe the target os/arch:
+
+| Env Variable                | Description                               |
+|-----------------------------|-------------------------------------------|
+| `CNB_TARGET_OS`             | Target OS                                 |
+| `CNB_TARGET_ARCH`           | Target architecture                       |
+| `CNB_TARGET_ARCH_VARIANT`   | Target architecture variant (optional)    |
+| `CNB_TARGET_DISTRO_NAME`    | Target OS distribution name (optional)    |
+| `CNB_TARGET_DISTRO_VERISON` | Target OS distribution version (optional) |
+
+Buildpacks can use this information to modify their behavior depending on the target.
+
+### Run image extensions is supported (experimental)
+
+In Platform 0.12, extensions can be used to extend not only build-time base images, but runtime base images as well.
+
+For more information, see [authoring an image extension](/docs/extension-guide/create-extension).