Document Swift and CocoaPods specifications in README.md

What does this MR do?

This documents links to important details about version syntax for both version strings and version ranges. For example, CocoaPods uses the Rubygems syntax for version ranges, and adheres to SemVer 2.0.0 version strings.

What are the relevant issue numbers?

n/a

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for this project, if necessary
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Security reports checked/validated by reviewer