Update README post-restructure of skills into separate installable plugins

[dbeatty10]

After #65 is merged, update README with callouts and instructions for anyone that's already installed these skills when they were all combined in the same plugin.

[b-per]

So, after merging I didn't see any error so far.

• In Claude Code, the dbt plugin was still installed, and updating it didn't raise errors
• At first the marketplace still showed one plugin, but after updating it showed 2
• I was still able at that time to update the dbt plugin
• And now I can see the dbt-migration plugin as well

I also ran a npx skills update and didn't see any error with it