Update PLUGIN.md

Author: pouyayarandi

Documentation/PLUGIN.md

@@ -182,6 +182,25 @@ exposed via public API, so even if `ImplementationOnlyImports` is set to `true`,
 this will only work if the `Visibility` is set to `internal`.
 
 
+##### Generation Option: `Applying` - Apply unsafe values to proto models
+
+Using this option, all generated proto models will conform to `_ApplyingProtocol`.
+This protocol requires `func applying(_ value, fieldNumber)` which enables us modify
+a field's value only by it's `fieldNumber`. For instance, if this option is enabled,
+it's possible to make a copy of a particular message with modifiying some of its 
+fields using the code below.
+
+```swift
+let new = try message
+  .applying(value1, for: 1)
+  .applying(value2, for: 2)
+```
+
+This code creates a new model which some of its fields (mapped fields 1 and 2 in
+proto model) has been modified. Notice that this is a throwing function and if 
+there's any problem with the applying, it will throw a `ProtoApplyingError` error.
+
+
 ### Building your project
 
 After copying the `.pb.swift` files into your project, you will need