add postmortem template from the Real-world SRE book

templates/postmortem-template-real-world-sre.md

+> Template from the [Real-world SRE](http://a.co/d/34UsUVU) book.
+
+# Title of the incident
+
+Name(s):
+
+Date: 
+
+Last modified: 
+
+## Summary
+
+## Impact
+
+## Timeline
+
+## Root Cause(s)
+
+## Action Items
+
+## Appendix