docs: replace data-testid using configure rather than custom render (#230)
docs: replace data-testid using configure rather than custom render (#230)

**What**:
Update documentation to suggest using `configure()` instead of writing a custom render, when needing to override `data-testid`.

**Why**:
We've just added a handy `configure()` function to `dom-testing-library` to make it easier to do this, so we might as well point it out here instead of recommending the hard way.

**How**:
Updated `README.md`

**Checklist**:

<!-- add "N/A" to the end of each line that's irrelevant to your changes -->

<!-- to check an item, place an "x" in the box like so: "- [x] Documentation" -->

- [x] Documentation
- [ ] Tests - N/A - this is only a docs change
- [x] Ready to be merged
- [ ] Added myself to contributors table - N/A - already there for docs

testing-library/react-testing-library,testing-library/react-testing-library,testing-library/react-testing-library