Update README.md

[Steve Mullerworth (stevemullerworth)]

Description

Updating readme in preparation for making repo public with documentation. Code will follow later.

Linked issues

Closes # (issue)

Developer tags

Type of change

• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• This change requires a documentation update.

How has this been tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration.

• New tests have been added.
• Tests have been modified to accommodate this change.
• GitHub workflows have been changed or added.

Checklist:

• The change has been checked for design compliance by an experienced SSE
  
• My code follows the style guidelines of this project.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes, for both debug and optimised builds.
• No AI tools have been used in the creation of this change.

Reasoning on why any of the above boxes have not been checked

Review Checks (To be filled in by the reviewer/s)

• Has the developer completed the appropriate sections above?
• Is the change compliant with LFRic Core principles?
• Is the testing coverage sufficient?
• Have any technical debt workarounds identified had issues opened and interested parties notified?

[Yaswant Pradhan (yaswant)]

Minor clarification about the role of trunk branch in the repo.

[Matthew Hambley (MatthewHambley)]

A small clarification.