[Twisted-Python] Responding to PRs

[Itamar Turner-Trauring]

Hi all,

Since we're starting to get PRs from random people it's worth trying to 
make the process as friendly as possible.

So, maybe instead of telling new contribtutors "PRs won't be reviewed 
without an issue, see contributor guidelines" it would be better to say 
"Thanks for the PR! I opened an issue for this PR here (tm.tl/12345). In 
the future we'd appreciate it if you could open an issue before 
submitting PRs; see contributor guidelines for details".

If someone went out of their way to provide a fix, we should try to 
minimize any unnecessary stop-energy they encounter along the way, even 
if that means some people won't learn the intricacies of the process.

-Itamar