docs: update readme.

Author: ghosind

README.md

@@ -1 +1,63 @@
-# go-optional
+# go-optional
+
+![test](https://github.com/ghosind/go-optional/workflows/test/badge.svg)
+[![Go Report Card](https://goreportcard.com/badge/github.com/ghosind/go-optional)](https://goreportcard.com/report/github.com/ghosind/go-optional)
+[![codecov](https://codecov.io/gh/ghosind/go-optional/branch/main/graph/badge.svg)](https://codecov.io/gh/ghosind/go-optional)
+![Version Badge](https://img.shields.io/github/v/release/ghosind/go-optional)
+![License Badge](https://img.shields.io/github/license/ghosind/go-optional)
+[![Go Reference](https://pkg.go.dev/badge/github.com/ghosind/go-optional.svg)](https://pkg.go.dev/github.com/ghosind/go-optional)
+
+A container object to describe the specified type value that may or may not contain a non-nil value.
+
+## Installation
+
+You can install the package by the following command.
+
+```sh
+go get -u github.com/ghosind/go-optional
+```
+
+## Getting Started
+
+You can create an `Optional` instance with a pointer, and perform actions with the `Optional` instance.
+
+```go
+s := "Hello world"
+vp := &s
+
+val = optional.New(vp)
+a.IsPresent() // true
+val.OrElse("default string") // Hello world
+val.IfPresent(func (s string) {
+  fmt.Println(s)
+})
+// Hello world
+
+// Optional instance with nil value
+vp = nil
+val := optional.New(vp)
+vp.IsPresent() // false
+val.OrElse("default string") // default string
+val.IfPresent(func (s string) {
+  fmt.Println(s)
+}) // Not invoked
+```
+
+The `Optional` type also supports marshaling the value to a JSON string, or unmarshal from a JSON string.
+
+```go
+type Example struct {
+  Val *optional.Optional[string] `json:"val"`
+}
+
+example := Example{
+  Val: optional.Of("Hello world"),
+}
+out, err := json.Marshal(example)
+fmt.Println(string(out))
+// {"val":"Hello world"}
+
+json.Unmarshal([]byte("Test"), &example)
+fmt.Println(example)
+// {Hello world}
+```