Create index.md

Added an error message for GHC-21231

Author: Soupstraw

message-index/messages/GHC-21231/index.md

@@ -0,0 +1,9 @@
+---
+title: Numeric escape sequence out of range
+summary: The numeric escape sequence represents a number that is too large
+introduced: 9.6.1
+severity: error
+---
+
+The escape sequence in the `Char` literal is not a valid unicode character. The maximum value of a numeric literal is \1114111 
+(or \x10ffff in hexadecimal and \o4177777 in octal).