Added new ReadOnly keyword documentation.

blitz.mod/blitz.bmx

@@ -844,6 +844,11 @@ bbdoc: Denote a class, function or method as final
 keyword: "Final"
 End Rem
 
+Rem
+bbdoc: Denote a field as read only, where the value may only be set in its declaration or in the type constructor
+keyword: "ReadOnly"
+End Rem
+
 Rem
 bbdoc: Specify constraints on the types that can be used as arguments for a type parameter defined in a generic declaration
 keyword: "Where"