chore(ruby): Add README file for ruby wrapper (#663)

* chore(ruby): Add README file for ruby wrapper

Author: aakashanandg

spannerlib/wrappers/spannerlib-ruby/README.md

@@ -0,0 +1,126 @@
+# Spannerlib Ruby Wrapper
+
+This Ruby wrapper provides an interface for interacting with Google Cloud Spanner via native code, leveraging [FFI (Foreign Function Interface)](https://github.com/ffi/ffi) to connect Ruby with native libraries—such as C or Go (compiled via cgo). It is part of the [`go-sql-spanner`](https://github.com/googleapis/go-sql-spanner) project and enables Ruby applications to utilize high-performance, native Spanner features.
+
+## What Does This Wrapper Do?
+
+- **FFI Integration:** Bridges Ruby with native C (or Go via cgo) code by wrapping native functions using FFI.
+- **High Performance:** Provides access to Spanner's performance and reliability through idiomatic Ruby APIs.
+- **Mock Server & Testing:** Includes mock server and integration tests to validate functionality.
+
+## Getting Started
+
+### Prerequisites
+
+- Ruby (>= 2.6)
+- [Bundler](https://bundler.io/): `gem install bundler`
+- [FFI Gem](https://github.com/ffi/ffi) (included in Gemfile)
+- Go (if building native libraries from source)
+
+### Setup
+
+Install Ruby dependencies:
+
+```sh
+bundle install
+```
+
+## FFI & Native Integration
+
+This wrapper uses the [FFI Ruby gem](https://github.com/ffi/ffi) to load and call native functions implemented in C (or Go via cgo).
+
+FFI setup is located in [`lib/spannerlib/ffi.rb`](lib/spannerlib/ffi.rb).  
+Refer to the source code [`lib/spannerlib/ffi.rb`](lib/spannerlib/ffi.rb) for details on function signatures and internals.
+
+## Running Tests
+
+### Mock Server Tests
+
+These tests run against an in-memory GRPC mock server.
+
+```sh
+bundle exec rspec spec/spannerlib_ruby_spec.rb
+```
+
+### Integration Tests
+
+Integration tests are run against the [Google Spanner Emulator](https://cloud.google.com/spanner/docs/emulator).
+
+Start the emulator using Docker (recommended):
+
+```sh
+docker run --rm -d \
+  -p 9010:9010 \
+  gcr.io/cloud-spanner-emulator/emulator
+```
+
+Then set the environment variable to direct client library calls to the emulator:
+
+```sh
+export SPANNER_EMULATOR_HOST=localhost:9010
+bundle exec rspec spec/integration/
+```
+
+## Linting & Fixes
+
+To check and auto-correct Ruby code style:
+
+```sh
+bundle exec rubocop         # Checks lint
+bundle exec rubocop -A      # Auto-corrects where possible
+```
+
+If any linting issues remain, resolve those manually.
+
+## Contributing
+
+We welcome contributions! Please follow these steps:
+
+1. Fork and clone this repository.
+2. Install dependencies with `bundle install`.
+3. Build any necessary native libraries if required.
+4. Run linting and tests before submitting a PR.
+5. Use clear commit messages (e.g., `chore(ruby): ...`).
+
+Feel free to open issues or pull requests for improvements.
+
+## Troubleshooting
+
+- **FFI Load Errors:** Ensure required native libraries are built and in your library path.
+- **Missing Gems:** Run `bundle install`.
+- **Spanner Credentials Issues:** Set up [Application Default Credentials](https://cloud.google.com/docs/authentication/provide-credentials-adc), for example by setting the `GOOGLE_APPLICATION_CREDENTIALS` environment variable.
+
+---
+
+For overall project details, see the main [go-sql-spanner README](../../../README.md).
+
+---
+
+**Reference:**  
+For examples of adding new functionality inside the FFI layer, see [PR #655](https://github.com/googleapis/go-sql-spanner/pull/655).
+
+## Building and Packaging the Ruby Gem
+
+Building native binaries for the Ruby wrapper (`spannerlib-ruby`) and publishing them as a Ruby gem is automated via GitHub Actions.
+
+### How it Works
+
+- Native shared libraries for Linux, macOS, and Windows are cross-compiled using GitHub-hosted runners and cross-compilers.
+- The binaries are packaged as part of the Ruby gem.
+- The process is defined in the [`release-ruby-wrapper.yml`](../../../.github/workflows/release-ruby-wrapper.yml) workflow file:
+  - Three main stages:
+    1. **Compilation:** Builds platform-specific binaries for Linux, macOS, and Windows.
+    2. **Artifact Upload:** Compiled binaries are uploaded for later packaging.
+    3. **Gem Packaging and Publishing:** The gem is built from source and published to RubyGems automatically if the workflow is manually dispatched.
+
+Please see the [GitHub Actions workflow file](../../../.github/workflows/release-ruby-wrapper.yml) for full implementation details.
+
+---
+
+**Note:** If you want to manually build the gem locally for testing, you can run the following from within `spannerlib/wrappers/spannerlib-ruby`:
+
+```sh
+gem build spannerlib-ruby.gemspec
+```
+
+For publishing, refer to [RubyGems guides](https://guides.rubygems.org/publishing/) or use the automated workflow.