Add back the PR template.

.github/PULL_REQUEST_TEMPLATE.md

+**Customer scenario**
+
+What does the customer do to get into this situation, and why do we think this
+is common enough to address in Escrow.  (Granted, sometimes this will be
+obvious "Open project, VS crashes" but in general, I need to understand how
+common a scenario is)
+
+**Bugs this fixes:**
+
+(either VSO or GitHub links)
+
+**Workarounds, if any**
+
+Also, why we think they are insufficient for RC vs. RC2, RC3, or RTW
+
+**Risk**
+
+This is generally a measure our how central the affected code is to adjacent
+scenarios and thus how likely your fix is to destabilize a broader area of code
+
+**Performance impact**
+
+(with a brief justification for that assessment (e.g. "Low perf impact because no extra allocations/no complexity changes" vs. "Low")
+
+**Is this a regression from a previous update?**
+
+**Root cause analysis:**
+
+How did we miss it?  What tests are we adding to guard against it in the future?
+
+**How was the bug found?**
+
+(E.g. customer reported it vs. ad hoc testing)