Add documentation to README about how to override the components

Problem/Motivation

Single Directory Components allow for being overriden by copying the directory to the theme. It would be nice to have some direction in the README that addresses how to override the component in cases where a gov't agency has their own design system that is built on top of USWDS.

Acceptance Criteria

1. Steps to override the SDC are added to the README
2. Any concerns, caveats or limitations are called out in the README

Proposed resolution

Remaining tasks

Steps to reproduce