Update contributing rules and PR management

[Stas'M]

Hello all!
 
Recently I have created a Pull Request which updates the contributing rules and PR
management:
https://github.com/reactos/reactos/pull/4881
 
This PR is a result of discussion in the chat with Joachim Henze and Thomas
Faber regarding noreply e-mails being used by some GitHub contributors.
 
It has been experimentally verified that using anonymous GitHub e-mail does not allow to
deliver mail to the commit authors. GitHub does replace author's real email when `Keep
my email addresses private` option is enabled in the account's e-mail settings.
 
Additionally I have replaced old IRC links with the references to our chat.

Please send an approval review on GitHub if you are okay with the changes I made in this
PR. Suggestions are also welcome.
 
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