Create README.md

Author: franks883

Core ServiceNow APIs/GlideAggregate/Incident resolution percentile by assignment group/README.md

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+# Incident resolution percentile by assignment group
+
+## What this solves
+Leaders often ask for P50 or P90 of incident resolution time by assignment group. Out-of-box reports provide averages, but percentiles are more meaningful for skewed distributions. This utility computes configurable percentiles from incident resolution durations.
+
+## Where to use
+- Script Include callable from Background Scripts, Scheduled Jobs, or Flow Actions
+- Example Background Script is included
+
+## How it works
+- Uses `GlideAggregate` to get candidate groups with resolved incidents in a time window
+- For each group, queries resolved incidents ordered by resolution duration (ascending)
+- Picks percentile ranks (for example 0.5, 0.9) using nearest-rank method
+- Returns a simple object per group with count, average minutes, and requested percentiles
+
+## Configure
+- `WINDOW_DAYS`: number of days to look back (default 30)
+- `GROUP_FIELD`: field to group by (default `assignment_group`)
+- Percentiles array (for example `[0.5, 0.9]`)
+
+## References
+- GlideAggregate API  
+  https://www.servicenow.com/docs/bundle/zurich-api-reference/page/app-store/dev_portal/API_reference/GlideAggregate/concept/c_GlideAggregateAPI.html
+- GlideRecord API  
+  https://www.servicenow.com/docs/bundle/zurich-api-reference/page/app-store/dev_portal/API_reference/GlideRecord/concept/c_GlideRecordAPI.html
+- GlideDateTime API  
+  https://www.servicenow.com/docs/bundle/zurich-api-reference/page/app-store/dev_portal/API_reference/GlideDateTime/concept/c_GlideDateTimeAPI.html