MyFirstContribution: mention trimming quoted text in replies

ReviewingGuidelines already advises reviewers to trim irrelevant quoted
context when replying. Give the same advice to new contributors in
MyFirstContribution, so our documentation is consistent about mailing
list reply etiquette.

Signed-off-by: Weijie Yuan <wy@wyuan.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: bertyuan

Documentation/MyFirstContribution.adoc

@@ -1367,6 +1367,11 @@ effect which had not occurred to you. It is always okay to ask for clarification
 if you aren't sure why a change was suggested, or what the reviewer is asking
 you to do.
 
+When replying to review comments, quote only the parts of the message that are
+relevant to your response. It is usually helpful to trim away unrelated context,
+such as large portions of the patch that are not being discussed, while keeping
+enough quoted text for readers to understand what you are responding to.
+
 Make sure your email client has a plaintext email mode and it is turned on; the
 Git list rejects HTML email. Please also follow the mailing list etiquette
 outlined in the