chore: Add archive notice to README

[dpk404]

Description

This pull request updates the README.md to announce that the Next CRM repository is being archived. It provides context for the discontinuation, migration guidance, and information about related projects.

Repository status and migration guidance:

• Added a prominent warning that Next CRM is being archived and will become read-only, with no further development or issue tracking.
• Explained the reasons for discontinuation, including the maturity of Frappe CRM, maintenance difficulties, and the planned deprecation of ERPNext CRM DocTypes.
• Provided instructions and a link to a migration tool for moving data to Frappe CRM, and encouraged users to use Frappe CRM as the new standard.
• Shared information about an extension app (Frappe CRM XT) for additional features and ongoing contributions to the upstream project.

Relevant Technical Choices

Testing Instructions

Additional Information:

Screenshot/Screencast

Checklist

• I have carefully reviewed the code before submitting it for review.
• This code is adequately covered by unit tests to validate its functionality.
• I have conducted thorough testing to ensure it functions as intended.
• A member of the QA team has reviewed and tested this PR (To be checked by QA or code reviewer)

Fixes #