doc(v3): add first draft of migration guide with links to it

Author: scotthart

CHANGELOG.md

@@ -1,5 +1,8 @@
 # Changelog
 
+**NOTE**: Please refer to the [V3 Migration Guide](doc/v3-migration-guide.md) 
+for details on updating existing applications using v1.x.y or v2.x.y.
+
 **NOTE**: [Future Breaking Changes](/doc/deprecated.md) lists anticipated
 breaking changes in the upcoming 3.x release. This release is scheduled for
 2024-12 or 2025-01.

README.md

@@ -10,6 +10,9 @@
 This repository contains idiomatic C++ client libraries for the following
 [Google Cloud Platform](https://cloud.google.com/) services.
 
+> Please refer to the [V3 Migration Guide](doc/v3-migration-guide.md) for
+> details on updating existing applications using v1.x.y or v2.x.y.
+
 > Please check the [CHANGELOG] for important announcements and upcoming changes.
 
 ## Quickstart

doc/v3-migration-guide.md

@@ -0,0 +1,53 @@
+# `google-cloud-cpp` v3 Migration Guide
+
+This document is intended for users of previous major versions (v1.x.y, v2.x.y)
+of the `google-cloud-cpp` SDK who are moving to a release on the v3.x.y series.
+
+While this repository does not strictly follow semver, it does use the major
+version number to indicate large breaking changes. We strive to balance the
+frequency in which we introduce breaking changes with improvements to the SDK.
+Since our most recent major version increment, about 3 years ago, we have added
+new API surfaces that supersede the previous deprecated types and functions. As
+part of the v3 release series, we are decommissioning those deprecated API
+surfaces. This guide serves a central location to document how to migrate from
+the decommissioned API surfaces to their replacements.
+
+## C++17
+
+Depending on your build system of choice, you should set the appropriate flag
+for your compiler if it does not already default to `--std=c++17` or higher.
+
+## Bazel Central Registry
+
+Bazel is moving away from WORKSPACE file support to using modules from the Bazel
+Central Registry. Part of the v3.x.y release series includes supporting the new
+[google-cloud-cpp](https://registry.bazel.build/modules/google_cloud_cpp) Bazel
+module which can be added to your MODULE.bazel file as a dependency.
+
+## Dependencies
+
+### Previously Optional Dependencies that are now Required
+
+- libcurl
+- nlohmann_json
+- opentelemetry-cpp
+
+### Relocated Dependencies
+
+- crc32c
+
+## Decommissioned API Surfaces
+
+### Bazel
+
+### CMake
+
+### Common
+
+### Bigtable
+
+### Pubsub
+
+### Spanner
+
+### Storage