update readme: add informations about the default tag option usage

Author: zak905

README.md

@@ -87,7 +87,32 @@ The supported field types in the struct are:
 
 Unsupported types are simply ignored, however custom types can be registered to be converted.
 
-More examples are available on the Gorilla website: https://www.gorillatoolkit.org/pkg/schema
+## Setting Defaults
+
+It is possible to set default values when encoding/decoding by using the `default` tag option. The value of `default` is applied when a field has a zero value, a pointer has a nil value, or a slice is empty.
+
+```go
+type Person struct {
+    Phone string `schema:"phone,default:+123456"`          // custom name
+    Age int     `schema:"age,default:21"`
+	Admin bool    `schema:"admin,default:false"`
+	Balance float64 `schema:"balance,default:10.0"`
+    Friends []string `schema:friends,default:john|bob`
+}
+```
+
+The `default` tag option is supported for the following types:
+
+* bool
+* float variants (float32, float64)
+* int variants (int, int8, int16, int32, int64)
+* uint variants (uint, uint8, uint16, uint32, uint64)
+* string
+* a slice of the above types. As shown in the example above, `|` should be used to separate between slice items. 
+* a pointer to one of the above types (pointer to slice and slice of pointers are not supported).
+
+> [!NOTE]  
+> Because primitive types like int, float, bool, unint and their variants have their default (or zero) values set by Golang, it is not possible to distinguish them from a provided value when decoding/encoding form values. In this case, the value provided by the `default` option tag will be always applied. For example, let's assume that the value submitted in the form for `balance` is `0.0` then the default of `10.0` will be applied, even if `0.0` is part of the form data for the `balance` field. In such cases, it is highly recommended to use pointers to allow schema to distinguish between when a form field has no provided value and when a form has a value equal to the corresponding default set by Golang for a particular type. If the type of the `Balance` field above is changed to `*float64`, then the zero value would be `nil`. In this case, if the form data value for `balance` is `0.0`, then the default will not be applied.
 
 ## License