MyFirstContribution: Document --range-diff option when writing v2

MyFirstContribution: Document --range-diff option when writing v2

In the "Sending v2" section, readers are directed to create v2 patches
without using --range-diff. However, it is customary to include a
range-diff against the v1 patches as a reviewer aid.

Update the "Sending v2" section to suggest a simple workflow that uses
the --range-diff option. Also include some explanation for -v2 and
--range-diff to help the reader understand the importance.

Signed-off-by: Glen Choo <redacted>
Signed-off-by: Junio C Hamano <redacted>