Add README for Date Difference snippet

This README provides an example and details on how to calculate the difference in days between two date/time values using GlideDateTime.

Author: harishkSN

Server-Side Components/GlideDateTime/DateCalculations/DateDifference/README.md

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+# Date Difference Snippet
+
+**Author:** Harish  
+**Category:** GlideDateTime / Date Calculations  
+**Use Case:** Calculate the difference in days between two date/time values.
+
+## Description
+This snippet uses `GlideDateTime` and `gs.dateDiff()` to calculate the number of days between two given date/time strings.
+
+## Example
+```js
+gs.info(getDateDifferenceInDays('2025-10-01 00:00:00', '2025-10-16 00:00:00'));