geotiffimage: stop mentioning that readRGB() forces interleaving

[sguimmara]

This PR fixes documentation that mention that readRGB() always interleave pixels (it does not if interleave: false in the options).

There is even an existing test case that proves it:

  it('should read into non-interleaved arrays if requested', async () => {
    const tiff = await GeoTIFF.fromSource(createSource('rgb.tiff'));
    const image = await tiff.getImage();
    const data = await image.readRGB({ ...options, interleave: false });
    expect(data).to.have.lengthOf(3);
    expect(data[0]).to.have.lengthOf(50 * 50);
    expect(data[1]).to.have.lengthOf(50 * 50);
    expect(data[2]).to.have.lengthOf(50 * 50);
  });

fixes #424

[TheMrCam]

LGTM beyond my one pedantic suggestion for the README

[constantinius]

Looks good to me. I'll merge this with @TheMrCam 's suggestion