Add PULL_REQUEST_TEMPLATE.md
Add PULL_REQUEST_TEMPLATE.md

Hopefully it will guide people wanting to contribute to the mailing
list.

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <73c5d9c9884d8595b466412486494d6aa45d1d55.1524476490.git.a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla