update readme to state LRUD in maintenance mode

[kukulaka]

Share information with community that LRUD is now in maintenance mode.

Description

Update readme.

Motivation and Context

Most internal projects now use lrud-spatial.

How Has This Been Tested?

Does not need test as just readme update.

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.