chore: Add SDK versioning policy

Sources/Core/AWSSDKForSwift/Documentation.docc/versioning.md

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+# Versioning Policy
+
+## Numbering
+
+The AWS SDK for Swift uses the following versioning format:
+```
+<major>.<minor>.<patch>
+```
+
+The version components are incremented when one of the following occurs:
+- Major: Significant, breaking API changes.  New major versions will be
+  very infrequent.  Expect to perform additional development to upgrade
+  to a new major version.
+- Minor: Changes that are not major but may require attention from
+  developers.  Specific changes will be detailed in release notes.
+  Examples of minor changes may be:
+  - Changes to behavior of existing features.
+  - Deprecation of existing SDK features.
+  - Dropping support for an Xcode, Swift, or platform version.
+  - A significant internal change to an existing feature.
+- Patch: Less significant changes that should require no additional development to adopt and should not change SDK behavior.
+  Patch releases will contain updates to the APIs for the AWS services.
+  There is an unlikely possibility that patch changes to specific AWS services may break existing APIs; please file an
+  [issue](https://github.com/awslabs/aws-sdk-swift/issues/new/choose)
+  if this occurs.
+  New patch versions are typically released each business day, with exceptions for holidays and certain AWS events.
+
+## AWS Service Client Libraries
+
+The AWS SDK for Swift includes a library for every AWS service (i.e. S3,
+EC2, etc) that includes the service client, model types, and other
+support code for accessing that AWS service.  Updating the SDK version
+updates all of the service libraries that are in use, along with all of
+the supporting runtime components.  In the future, we may elect to
+publish services separately.
+
+## Public APIs
+
+The AWS SDK for Swift consists of several modules which will
+automatically install at build time.  The interfaces between these
+modules are necessarily public, but not all are intended for use by
+customers and may be altered in the future.  AWS SDK for Swift uses the
+"System Programming Interface" (or SPI) feature to mark API features
+that are not meant for customers to use.  If you find yourself needing
+to access a public interface marked with `@_spi`, please start a
+[discussion](https://github.com/awslabs/aws-sdk-swift/discussions/new/choose)
+and the SDK developers will work with you to find a solution.