Update issue templates

.github/ISSUE_TEMPLATE/epic.md

+---
+name: Epic
+about: Use this template to create a new epic for the sprint backlog
+title: Describe the epic in a few keywords
+labels: ''
+assignees: ''
+
+---
+
+# Epic
+<!--  
+This is the template to create an epic. An epic contains a user story as description and a background. Epics are broken down in user-stories (when possible or when there are some sub-tasks or when the implementation is not clear yet) or tasks (when there are no sub-tasks, and implementation is clear).
+-->
+
+## Story
+<!--  
+Your user story should be in the form:  As a <user> I want <feature> such that <benefit of feature/product increment>`
+A user can be anybody who interacts with the Swarm code (node operator/app developer/client developer).
+-->
+
+## Background
+<!--  
+Mention here the reason of the user story, any technical details, link to meeting notes, architecture diagrams, etc. 
+-->