DEVX-6966: Append to user agent

[superchilled]

Description

This PR does the following:

• Updates the README.md file to:
  • Provide guidance on appending a custom string to the User-Agent header

Motivation and Context

We need to add the ability to append to the user agent in the OpenTok SDKs. This is so the partner's team can track usage.

The Ruby SDK already provides this functionality, but it wasn't previously documented in the README. This PR fixes that.

How Has This Been Tested?

There is already a unit test for this behaviour.

Example Output or Screenshots (if appropriate):

n/a

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.