Added date timeline

Added revision to the timeline in the front

Author: markweitzel

azure/Guidelines.md

@@ -3,7 +3,8 @@
 
 | Date        | Notes                                                          |
 | ----------- | -------------------------------------------------------------- |
-| 2022-Feb-01 | Updated error guidance                                        |
+| 2022-Mar-25 | Update guideline for date values in headers to follow RFC 7231 |
+| 2022-Feb-01 | Updated error guidance                                         |
 | 2021-Sep-11 | Add long-running operations guidance                           |
 | 2021-Aug-06 | Updated Azure REST Guidelines per Azure API Stewardship Board. |
 | 2020-Jul-31 | Added service advice for initial versions                      |