docs: add readme (#7)

* Add a README file.

* Improve documentation on some variable and type.

README.md

@@ -0,0 +1,45 @@
+# graceful
+
+[![Run Tests](https://github.com/gin-contrib/graceful/actions/workflows/go.yml/badge.svg?branch=master)](https://github.com/gin-contrib/graceful/actions/workflows/go.yml)
+[![codecov](https://codecov.io/gh/gin-contrib/graceful/branch/master/graph/badge.svg)](https://codecov.io/gh/gin-contrib/graceful)
+[![Go Report Card](https://goreportcard.com/badge/github.com/gin-contrib/graceful)](https://goreportcard.com/report/github.com/gin-contrib/graceful)
+[![GoDoc](https://godoc.org/github.com/gin-contrib/graceful?status.svg)](https://godoc.org/github.com/gin-contrib/graceful)
+[![Join the chat at https://gitter.im/gin-gonic/gin](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/gin-gonic/gin)
+
+Gin wrapper to enable graceful termination when shutting down a process
+
+## Example
+
+```go
+package main
+
+import (
+	"context"
+	"net/http"
+	"os/signal"
+	"syscall"
+
+	"github.com/gin-contrib/graceful"
+	"github.com/gin-gonic/gin"
+)
+
+func main() {
+	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer stop()
+
+	router, err := graceful.Default()
+	if err != nil {
+		panic(err)
+	}
+	defer router.Close()
+
+	router.GET("/", func(c *gin.Context) {
+		c.String(http.StatusOK, "Welcome Gin Server")
+	})
+
+	if err := router.RunWithContext(ctx); err != nil && err != context.Canceled {
+		panic(err)
+	}
+}
+```
+

graceful.go

@@ -10,7 +10,7 @@ import (
 	"github.com/gin-gonic/gin"
 )
 
-// Graceful is a wrapper around a gin.Engine that provides graceful shutdown
+// Graceful is a wrapper around a [gin.Engine] that provides graceful shutdown
 type Graceful struct {
 	*gin.Engine
 
@@ -23,7 +23,10 @@ type Graceful struct {
 	cleanup        []cleanup
 }
 
+// ErrAlreadyStarted is returned when trying to start a router that has already been started
 var ErrAlreadyStarted = errors.New("already started router")
+
+// ErrNotStarted is returned when trying to stop a router that has not been started
 var ErrNotStarted = errors.New("router not started")
 
 type listenAndServe func() error