(7.x) Introduce PR template to remind contributors all the tasks they need to do

Introduce PR template for https://github.com/jbossas/jboss-eap7/ to remind contributors all the tasks they need to do.

Details about PR template: https://github.com/blog/2111-issue-and-pull-request-templates
Example of template in use: https://github.com/wildfly-swarm/wildfly-swarm/pull/219

Template should reflect what's written in https://github.com/jbossas/jboss-eap7/blob/7.x/CONTRIBUTING.md

Thing to consider: requirements for template can differ for 7.x and 7.0.z