doc updated

README.md

@@ -1,2 +1,93 @@
 # summoner
 Typeclass system and dependency injection for Golang
+
+## Basic dependency injection
+You can use `summoner` to achieve simple typeclass function.
+For example, if you want to have a typeclass `Show`, 
+and implement it for types `int`, `string` and `Person`:
+```go
+type Show[A any] interface {
+	Show(a A) string
+}
+
+type ShowInt struct{}
+func (self *ShowInt) Show(i int) string {
+    fmt.Printf("Show int %v", i)
+}
+
+type ShowString struct{}
+func (self *ShowString) Show(s string) string {
+    fmt.Printf("Show str %v", s)
+}
+
+type Person struct {
+    id int
+    name string
+    age int
+}
+type ShowPerson struct{}
+func (self *ShowPerson) Show(p Person) string {
+    fmt.Printf("Show person %v", p)
+}
+
+
+func main() {
+    Given[Show[int]](new(ShowInt))
+    Given[Show[int]](new(ShowString))
+    Given[Show[int]](new(ShowPerson))
+
+    si, err := Summon[Show[int]]()
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(si.Show(7))
+
+    ss, err := Summon[Show[String]]()
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(ss.Show("str"))
+
+    sp, err := Summon[Show[Person]]()
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(sp.Show(Person{0, "Tom", 10}))
+}
+```
+
+## Summon rules to summon inferred value
+Suppose you want to build instance from some existed instance,
+for example, 
+- *if a type has implemented typeclass `Show`, then it should implement `Debug` automatically* or
+- *For all types implemented `Show`, they also implement `Debug` dynamically*
+Here's the code:
+```go
+type Debug[A any] struct {
+	Show Show[A] // If implemented `Show`, then implement `Debug`
+}
+
+func (self *Debug[A]) Debug(a A) string {
+	return fmt.Sprintf("Debug: %s", self.Show.Show(a))
+}
+
+
+func main() {
+    Given[Show[int]](new(ShowInt))
+    Given[Show[int]](new(ShowString))
+    Given[Show[int]](new(ShowPerson))
+
+    si, err := Summon[Debug[int]]() // Get Debug[int] for free
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(si.Debug(7))
+
+    sp, err := Summon[Debug[Person]]() // Get Debug[Person] for free
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(sp.Debug(Person{0, "Tom", 10}))
+}
+
+```