Add migration guide (oras-project#192)

Signed-off-by: Lixia (Sylvia) Lei <[email protected]>

MIGRATION_GUIDE.md

@@ -0,0 +1,45 @@
+# Migration Guide
+
+In version `v2`, ORAS Go library has been completely refreshed with:
+
+- More unified interfaces
+- Notably fewer dependencies
+- Higher test coverage
+- Better documentation
+
+**Besides, ORAS Go `v2` is now a registry client.**
+
+## Major Changes in `v2`
+
+- Moves `content.FileStore` to [file.Store](https://pkg.go.dev/oras.land/oras-go/v2/content/file#Store)
+- Moves `content.OCIStore` to [oci.Store](https://pkg.go.dev/oras.land/oras-go/v2/content/oci#Store)
+- Moves `content.MemoryStore` to [memory.Store](https://pkg.go.dev/oras.land/oras-go/v2/content/memory#Store)
+- Provides [SDK](https://pkg.go.dev/oras.land/oras-go/v2/registry/remote) to interact with OCI-compliant and Docker-compliant registries
+- Supports [Copy](https://pkg.go.dev/oras.land/oras-go/v2#Copy) with more flexible options
+- Supports [Extended Copy](https://pkg.go.dev/oras.land/oras-go/v2#ExtendedCopy) with options *(experimental)*
+- No longer supports `docker.Login` and `docker.Logout` (removes the dependency on `docker`); instead, provides authentication through [auth.Client](https://pkg.go.dev/oras.land/oras-go/v2/registry/remote/auth#Client)
+
+Documentation and examples are available at [pkg.go.dev](https://pkg.go.dev/oras.land/oras-go/v2).
+
+## Migrating from `v1` to `v2`
+
+1. Get the `v2` package
+
+    ```sh
+    go get oras.land/oras-go/v2
+    ```
+
+2. Import and use the `v2` package
+
+    ```go
+    import "oras.land/oras-go/v2"
+    ```
+
+3. Run
+
+   ```sh
+   go mod tidy
+    ```
+
+Since breaking changes are introduced in `v2`, code refactoring is required for migrating from `v1` to `v2`.  
+The migration can be done in an iterative fashion, as `v1` and `v2` can be imported and used at the same time.

README.md

@@ -44,6 +44,8 @@ If you are seeking new features over stability, you should use the
 `main` branch (or a specific commit hash) when including the ORAS
 Go library in your project's `go.mod`.
 
+To migrate from `v1` to `v2`, see [MIGRATION_GUIDE.md](./MIGRATION_GUIDE.md).
+
 ## Docs
 
 Documentation for the ORAS Go library is located on