Add buildpack migration guide (stub)

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

Author: Natalie Arellano

content/docs/reference/spec/migration/buildpack-api-0.8-0.9.md

@@ -0,0 +1,29 @@
++++
+title="Buildpack API 0.8 -> 0.9"
++++
+
+<!--more-->
+
+This guide is most relevant to buildpack authors.
+
+See the [spec release](https://github.com/buildpacks/spec/releases/tag/buildpack%2Fv0.9) for buildpack API 0.9 for the full list of changes and further details.
+
+### Shell removal
+
+Buildpack-defined processes may no longer implicitly rely on a shell (be non-direct). 
+
+In `launch.toml`, `direct` is removed as a key in the `[[processes]]` table, and all processes are inferred to be `direct = true`.
+
+Buildpack processes can still use a shell! However, the `command` must now explicitly begin with `/bin/sh` (or `cmd.exe` on Windows). 
+
+### Overridable process arguments
+
+Hand-in-hand with shell removal is the introduction of overridable process arguments.
+
+In `launch.toml`, `command` is now a list. The first element in `command` is the command, and all following entries are arguments that are always provided to the process, regardless of how the application is started. The `args` list now designates arguments that can be overridden by the end user - if supported by the platform (platform API version 0.10 and above). For further details, see the platform [migration guide][TODO].
+
+For older platforms (platform API version 0.9 and below), arguments in `args` will be appended to arguments in `command`, negating the new functionality (but preserving compatibility).
+
+### Image extensions are supported (experimental)
+
+TODO

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

@@ -10,7 +10,7 @@ See the [spec release](https://github.com/buildpacks/spec/releases/tag/platform%
 
 ## Platform Operator
 
-### The `launcher` supports overridable process args
+### The `launcher` supports overridable process arguments
 
 The way user-provided arguments are handled by the launcher depends on the buildpack API of the buildpack that created the process definition.
 
@@ -48,7 +48,7 @@ command = ["some-command"]
 args = ["always-1", "always-2"]
 ```
 
-The `command` list will never have more than one entry. `always-1` and `always-2` are arguments that are always provided to `some-command`. If no user-provided arguments are specified when the application image is launched, `always-1` and `always-2` will be provided only. If user-provided arguments are specified, these will be **appended** to the `args` list. Example:
+The `command` list will never have more than one element. `always-1` and `always-2` are arguments that are always provided to `some-command`. If no user-provided arguments are specified when the application image is launched, `always-1` and `always-2` will be provided only. If user-provided arguments are specified, these will be **appended** to the `args` list. Example:
 
 ```
 docker run --entrypoint from-newer-buildpack my-image