Docs: state minimum Julia version

[Andy Nowacki]

In the docs, state the minimum version of Julia which is required for the package.

[Charlotte Haley]

I’ve added this here. Thanks.

[Andy Nowacki]

Just to clarify, Project.toml says you need Julia >= v1.3, but the README.md says v1.4.2, so it would be good to make them consistent.

[Charlotte Haley]

This commit addresses the discrepancy, thanks.