MyFirstContribution: refrain from self-iterating too much

Finding mistakes in and improving your own patches is a good idea,
but doing so too quickly is being inconsiderate to reviewers who
have just seen the initial iteration and taking their time to review
it.  Encourage new developers to perform such a self review before
they send out their patches, not after.  After sending a patch that
they immediately found mistakes in, they are welcome to comment on
them, mentioning what and how they plan to improve them in an
updated version, before sending out their updates.

Helped-by: Torsten Bögershausen <tboegi@web.de>
Helped-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/MyFirstContribution.txt

@@ -1256,6 +1256,38 @@ index 88f126184c..38da593a60 100644
 [[now-what]]
 == My Patch Got Emailed - Now What?
 
+Please give reviewers enough time to process your initial patch before
+sending an updated version. That is, resist the temptation to send a new
+version immediately, because others may have already started reviewing
+your initial version.
+
+While waiting for review comments, you may find mistakes in your initial
+patch, or perhaps realize a different and better way to achieve the goal
+of the patch. In this case you may communicate your findings to other
+reviewers as follows:
+
+ - If the mistakes you found are minor, send a reply to your patch as if
+   you were a reviewer and mention that you will fix them in an
+   updated version.
+
+ - On the other hand, if you think you want to change the course so
+   drastically that reviews on the initial patch would be a waste of
+   time (for everyone involved), retract the patch immediately with
+   a reply like "I am working on a much better approach, so please
+   ignore this patch and wait for the updated version."
+
+Now, the above is a good practice if you sent your initial patch
+prematurely without polish.  But a better approach of course is to avoid
+sending your patch prematurely in the first place.
+
+Please be considerate of the time needed by reviewers to examine each
+new version of your patch. Rather than seeing the initial version right
+now (followed by several "oops, I like this version better than the
+previous one" patches over 2 days), reviewers would strongly prefer if a
+single polished version came 2 days later instead, and that version with
+fewer mistakes were the only one they would need to review.
+
+
 [[reviewing]]
 === Responding to Reviews