add go mod

[sivchari]

go version                                                                                                                                                                                                                                                                                    
go version go1.16.6 darwin/arm64

go run example/main.go                                                                                                                                                                                                                                                                        
build command-line-arguments: cannot find module for path _/Users/sivchari/workspace/notificator

Since Go1.11, we cannot use relative path. So, this example is not working.

I added go.mod and fixed example.