message: Update README.md to note master and releases/stable branches
diff: Welcome to the MongoDB C++ Driver!
  +## Branches - `releases/stable` versus `master`
  +
  +The default checkout branch of this repository is `releases/stable`.
  +This will always contain the latest stable release of the driver. The
  + `master` branch is used for active development. `master` should
  +**only** be used when making contributions back to the driver, as it
  +is not stable for use in production.
  +
  +See [Driver Status by family and version](#driver-status-by-family-and-version)
  +for more details about the various versions of the driver.
  +
  ## Resources
  * [MongoDB C++ Driver Quickstart](https://mongodb.github.io/mongo-cxx-driver/mongocxx-v3/tutorial/)
  