Update README to include AddErrorMessage explanation

README.md

@@ -58,6 +58,7 @@ Valgo is used in production by [Statsignal](https://statsignal.dev), but we want
   - [`In(...)` function](#in-function)
   - [`InRow(...)` function](#inrow-function)
   - [`Check(...)` function](#check-function)
+  - [`AddErrorMessage(...)` function](#adderrormessage-function)
   - [Merging two `Validation` sessions with `Validation.Merge( ... )`](#merging-two-validation-sessions-with-validationmerge--)
   - [`New()` function](#new-function)
   - [Custom error message template](#custom-error-message-template)
@@ -310,6 +311,48 @@ output:
 }
 ```
 
+## `AddErrorMessage(...)` function
+
+The `AddErrorMessage` function allows to add an error message to a Validation session without executing a field validator. This function takes in two arguments: `name`, which is the name of the field for which the error message is being added, and `message`, which is the error message being added to the session.
+
+When an error message is added using this function, the Validation session is marked as invalid, indicating that at least one validation error has occurred.
+
+One use case for the `AddErrorMessage` function is to add a general error message for the validation of an entity structure. As shown in the example below, if you have an entity structure for an `address` and need to validate multiple fields within it, such as the `city` and `street`, you can use `AddErrorMessage` to include a general error message for the entire `address` in case any of the fields fail validation.
+
+```go
+type Address struct {
+  City string
+  Street string
+}
+
+a := Address{"", "1600 Amphitheatre Pkwy"}
+
+val := v.
+  Is(String(a.city, "city").Not().Blank()).
+  Is(String(a.Street, "street").Not().Blank())
+
+if !val.Valid() {
+  v.AddErrorMessage("address", "The address is wrong!")
+
+  out, _ := json.MarshalIndent(val.Error(), "", "  ")
+  fmt.Println(string(out))
+}
+
+```
+output:
+```json
+{
+  "address": [
+    "The address is wrong!"
+  ],
+  "city": [
+    "City can't be blank"
+  ]
+}
+```
+
+ It's worth noting that there may be other use cases for this function as well.
+
 ## Merging two `Validation` sessions with `Validation.Merge( ... )`
 
 Using `Merge(...)` you can merge two `Validation` sessions. When two validations are merged, errors with the same value name will be merged. It is useful for reusing validation logic.