doc: add cast and convert description

README.md

@@ -17,6 +17,8 @@ go get github.com/zeiss/pkg
 
 Go has a pretty good standard library, but there are some things that are missing. This collection of small packages is meant to fill those gaps.
 
+## Casting values
+
 There is the typical case of pointers you need to deference. For example in the strict interfaces generated by [oapi-codegen](https://github.com/oapi-codegen/oapi-codegen).
 
 ```go
@@ -40,17 +42,26 @@ SomeStruct {
 Return a default value of a `nil` pointer.
 
 ```go
+type Foo struct {}
+cast.Zero(Foo) // &Foo{}
+```
 
-func Value[T any](val *T) T {
-	if val == nil {
-		return Zero[T]()
-	}
+## Converting values
 
-	return *val
-}
+There is also always the case to covert a value to a specific other value.
 
+```go
+// String converts a value to a string.
+b := true
+s := cast.String(b)
+
+fmt.Println(s) // "true"
 ```
 
+There are functions to convert `int`, `string` and `bool` values.
+
+## Databases
+
 There are also more complex tools like the `Database` interface which enables to easliy implement database wrappers.
 
 ```go