Updating "contribution Checklist" to recommend git format-patch.

[Carlos O'Donell]

Community,

I want to make two changes to the contribution checklist:

(a) Remove:
~~~
This should not be part of the change diff; put it right before 
the diff. (The git-show-gnu script can be used to produce the 
standard format for posting a patch once you've done a git 
commit on a local branch.) 
~~~

I think we should just be having people generate patches with
`git format-patch` and send those inline or as attachments to
the list.

We should not require people to use git-show-gnu scripts, and
all developers should have the Changelog merge helper to make
it easy to commit such patches.

It will also make it abundantly clear what the commit message
is going to be.

Any objections?

Cheers,
Carlos.