Add migration guide

Signed-off-by: Mikey Boldt <[email protected]>

Author: mboldt

content/docs/reference/spec/migration/buildpack-api-0.7-0.8.md

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++++
+title="Buildpack API 0.7 -> 0.8"
++++
+
+<!--more-->
+
+This guide is most relevant to buildpack authors.
+
+See the [spec release](https://github.com/buildpacks/spec/releases/tag/buildpack%2Fv0.8) for buildpack API 0.8 for the full list of changes and further details.
+
+### Process-Specific Working Directory
+
+Buildpacks may specify the working directory for a process by setting the `working-dir` field on the process in [`launch.toml`](https://github.com/buildpacks/spec/blob/buildpack/0.8/buildpack.md#launchtoml-toml).
+
+Prior to this, all processes ran from the app directory (`CNB_APP_DIR`).
+Running a process from a different directory typically involved running a shell to execute a `cd` command and then start the process, like:
+
+```
+[[processes]]
+command = "bash"
+args = ["-c", "cd <working-dir> && <start-process>"]
+```
+
+Buildpacks can now specify the process directly with a specific working directory, like:
+
+```
+[[processes]]
+command = "<start-process>"
+working-dir = "<working-dir>"
+```
+
+For details, see [RFC 81](https://github.com/buildpacks/rfcs/blob/main/text/0081-process-specific-working-directory.md).
+
+### Deprecate Positional Args to `build` and `detect` Executables
+
+The positional arguments to the `detect` and `build` executables are deprecated.
+The lifecycle now provides these values as environment variables.
+
+To upgrade, buildpack authors should use the following environment variables:
+
+For `detect`:
+
+- `CNB_PLATFORM_DIR` replaces the first positional argument.
+- `CNB_BUILD_PLAN_PATH` replaces the second positional argument.
+
+For `build`:
+
+* `CNB_LAYERS_DIR` replaces the first positional argument.
+* `CNB_PLATFORM_DIR` replaces the second positional argument.
+* `CNB_BP_PLAN_PATH` replaces the third positional argument.
+
+For details, see [RFC 100](https://github.com/buildpacks/rfcs/blob/main/text/0100-buildpack-input-vars.md).