add vendor & update README.md

vendor/github.com/gin-contrib/sse/writer.go

@@ -0,0 +1,24 @@
+package sse
+
+import "io"
+
+type stringWriter interface {
+	io.Writer
+	WriteString(string) (int, error)
+}
+
+type stringWrapper struct {
+	io.Writer
+}
+
+func (w stringWrapper) WriteString(str string) (int, error) {
+	return w.Writer.Write([]byte(str))
+}
+
+func checkWriter(writer io.Writer) stringWriter {
+	if w, ok := writer.(stringWriter); ok {
+		return w
+	} else {
+		return stringWrapper{writer}
+	}
+}