Add version support information to the readme #827
Description
From #804 where the strategy for supporting the 1.x and 2.x versions of localgov_base were decided.
Add wording to the readme:
All new feature development work will be against the 2.x branch
The 1.x branch will continue to have bug fixes until the end of December 2025.
In some cases, if you have made customisations to the child theme, changes may be needed to the child theme to upgrade from 1.x to 2.x
Please see release notes for information https://github.com/localgovdrupal/localgov_base/releases