updated upgrade instructions

UPGRADE.md

@@ -1,14 +1,21 @@
 # Upgrade Instructions
 
 ## Upgrade from 3.x to 4.x
-* Change your error message placeholders from the Go `fmt` package `verbs` to the `template variables`. 
+
+If you are customizing the error messages of the following built-in validation rules, you may need to 
+change the parameter placeholders from `%v` to the Go template variable placeholders.
+* `Length`: use `{{.min}}` and `{{.max}}` in the error message to refer to the minimum and maximum lengths.
+* `Min`: use `{{.threshold}}` in the error message to refer to the lower bound.
+* `Max`: use `{{.threshold}}` in the error message to refer to the upper bound
+* `MultipleOf`: use `{{.base}}` in the error message to refer to the base of the multiples.
+
+For example,
  ```go
 // 3.x
-// Assume you want to set a custom message for the LengthRule:
-var lengthRule= validation.Length(2,10).Error("the length must be between %v and %v")
+lengthRule := validation.Length(2,10).Error("the length must be between %v and %v")
 
 // 4.x
-var lengthRule= validation.Length(2,10).Error("the length must be between {{.min}} and {{.max}}")
+lengthRule := validation.Length(2,10).Error("the length must be between {{.min}} and {{.max}}")
 ```
 
 ## Upgrade from 2.x to 3.x