readme: Add core UMaskExt documentation

This commit adds documentation for the core event attribute UMaskExt.
Note that the attribute will eventually be renamed to UMask2 to align
with the Intel SDM.

Author: edwarddavidbaker

README.md

@@ -199,9 +199,19 @@ is defined architecturally. Each value corresponds to an event logic unit and sh
 mask value to obtain an architectural performance event.
 
 ### UMask
-This field maps to the Unit Mask filed in the `IA32_PERFEVTSELx[15:8]` MSRs. It further qualifies the event logic
+This field maps to the Unit Mask field in the `IA32_PERFEVTSELx[15:8]` MSRs. It further qualifies the event logic
 unit selected in the event select field to detect a specific micro-architectural condition.
 
+### UMaskExt (Core events)
+This field maps to the Unit Mask 2 field in the `IA32_PERFEVTSELx[47:40]` MSRs. First introduced with architectural
+performance monitoring version 6.
+
+> These bits qualify the condition that the selected event logic unit detects. Valid UMASK2 values for each
+event logic unit are specific to the unit. The new UMASK2 field may also be used in conjunction with UMASK.
+
+:warning: `UMaskExt` will be renamed to `UMask2` to align with the Intel® SDM. Please refer to
+https://github.com/intel/perfmon/issues/357 for additional information.
+
 ### EventName
 It is a string of characters to identify the programming of an event.