Update README.md

[haf]

Improving documentation for how to override macros.

[clrcrl]

Reassigning to Jeremy as dispatch macros are his area of knowledge :)

[jtcohen6]

Thanks for this @haf!

Am I right in seeing this Slack thread as the context for the change here?

I left a few suggestions. It feels like there are two adjacent-yet-different use cases for leveraging dispatch:

1. Extending dbt_utils macros to new adapters, by reimplementing low-lying functionality
2. Overriding dbt_utils macros on existing adapters, by reimplementing low-lying functionality (and ensuring the custom version is used by other dbt_utils macros)

There's a lot of overlap between them in terms of the actual steps required. I'd like to keep the guidance and context as clear as possible; the reader is likely here because they want to do one or the other.

[jtcohen6]

Thanks for this @haf!