Add template for epic/story issues (#646)

This should help filling in some usual details

.github/ISSUE_TEMPLATE/epic-or-story.md

@@ -0,0 +1,60 @@
+---
+name: Epic/Story Issue Template
+about: Template for "top" level issues - Epics (>2 weeks) / Stories (<2 weeks)
+title: 'Epic: New Feature'
+labels: epic
+assignees:
+
+---
+
+## Summary / Background
+What do you want to achieve, why? business context
+...
+
+## Scope
+
+What will be impacted and what won't be?
+What needs implementation and what is invariant?
+e.g.
+- user should be able to run workflow X from UI
+- enable workflow Y, Z from CLI
+
+## Assumptions
+Product / UX assumptions as well as technical assumptions / limitations
+e.g.
+* Support only Python Runtime
+* Focus on DVC experiments only
+* Deployment environments don't change often and can be picked up from shared configuration
+
+## Open Questions
+e.g.
+- How should access control work for shared artifacts (workflow X)
+- Python runtime assumption - is it really valid? in light of <...>
+
+## Blockers / Dependencies
+List issues or other conditions / blockers
+
+## General Approach
+Invocation example:
+```shell
+$ mapper-run task.tar.gz --ray-cluster <ip>:<port>
+```
+
+## Steps
+
+### Must have (p1)
+- [ ] subissue2
+- [ ] step 2
+  - info
+  - info
+
+### Optional / followup (p2)
+- [ ] ⌛ step 3 wip
+- [ ] step 4
+
+## Timelines
+
+Put your estimations here. Update once certainty changes
+- end of week (Feb 3) for prototype with workflows X, Y
+- Feb 15 - MVP in production
+- Low priority followups can be done later