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>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,15 @@
+# Thank you for your contribution
+
+You are welcome to open PR, but they are used for discussion only. All
+patches must eventually go to the openvpn-devel mailing list for review:
+
+* https://lists.sourceforge.net/lists/listinfo/openvpn-devel
+
+Please send your patch using [git-send-email](https://git-scm.com/docs/git-send-email). For example to send your latest commit to the list:
+
+    $ git send-email --to=openvpn-devel@lists.sourceforge.net HEAD~1
+
+For details, see these Wiki articles:
+
+* https://community.openvpn.net/openvpn/wiki/DeveloperDocumentation
+* https://community.openvpn.net/openvpn/wiki/Contributing