docs: Added UPGRADING.md with admin client migration

Author: gkevinzheng

UPGRADING.md

@@ -0,0 +1,36 @@
+# 3.0.0 Migration Guide
+
+The v3.0.0 release of `google-cloud-bigtable` deprecates the previous `google.cloud.bigtable.Client` class in favor of distinct clients for the two API surfaces, supporting both sync and async calls:
+- Data API: 
+  - `google.cloud.bigtable.data.BigtableDataClient`
+  - `google.cloud.bigtable.data.BigtableDataClientAsync`
+- Admin API: 
+  - `google.cloud.bigtable.admin.BigtableInstanceAdminClient`
+  - `google.cloud.bigtable.admin.BigtableInstanceAdminClientAsync`
+  - `google.cloud.bigtable.admin.BigtableTableAdminClient`
+  - `google.cloud.bigtable.admin.BigtableTableAdminClientAsync`
+
+The deprecated client will remain available as an alternative API surface, which internally delegates calls to the respective new clients. For most users, existing code will continue to work as before. But there may be some breaking changes associated with this update, which are detailed in this document.
+
+### Admin Client Migration
+
+While we do recommend you instantiate the new admin API clients directly, you can access the new admin API 
+clients via the `google.cloud.bigtable.Client` class:
+
+```
+from google.cloud.bigtable import Client
+
+client = Client()
+
+
+# google.cloud.bigtable.admin.BigtableInstanceAdminClient
+instance_admin_client = client.instance_admin_client()
+
+# google.cloud.bigtable.admin.BigtableTableAdminClient
+table_admin_client = client.table_admin_client()
+```
+
+## Breaking Changes
+- **[MutationBatcher](https://github.com/googleapis/python-bigtable/blob/main/google/cloud/bigtable/data/_sync_autogen/mutations_batcher.py#L151)and [MutationBatcherAsync](https://github.com/googleapis/python-bigtable/blob/main/google/cloud/bigtable/data/_async/mutations_batcher.py#L182)'s `table` argument was renamed to `target`**, since it also supports [Authorized View](https://github.com/googleapis/python-bigtable/pull/1034) instances. This matches the naming used in other classes (PR: https://github.com/googleapis/python-bigtable/pull/1153)
+<!-- TODO: Replace Github link once the feature branch is merged -->
+- **[`BigtableTableAdminClient`/`BigtableTableAdminAsyncClient`](https://github.com/googleapis/python-bigtable/blob/main/google/cloud/bigtable_admin_v2/overlay/services/bigtable_table_admin)'s location was changed from `google.cloud.bigtable_admin_v2.services.bigtable_table_admin` to `google.cloud.bigtable_admin_v2.overlay.services.bigtable_table_admin`**. This should not affect you if you are importing these clients via `import google.cloud.bigtable_admin_v2` due to import aliasing, but will affect you if you are importing these clients via `import google.cloud.bigtable_admin_v2.services.bigtable_table_admin`.