github: Add PR template with contributor related information

github: Add PR template with contributor related information

Add PR template in order to simplify new developers cooperate properly if
they open PR. discussed here:
https://sourceforge.net/p/openvpn/mailman/message/35601310/

v2: Changed links and provided an example of "git send-email"

[DS: Cleaned up commit message, changed order of URLs at commit time]

Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <1486364115-9801-1-git-send-email-chipitsine@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14010.html
Signed-off-by: David Sommerseth <davids@openvpn.net>