expand (CD) acronym

What does this MR do?

This change expands an acronym in the README.md. The acronym CD is ambiguous and can mean many things. I chose to expand the acronym to continuous delivery/deployment since that terminology seems to be consistent with our documentation.

As per the writing style guidelines

Do not use acronyms when you can avoid it as you can't assume people know what you are talking about. Example: instead of MR, write merge request.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team