Add Go/golang example code

[KEINOS]

♻️ Current situation

• There is no Go (golang) example in the README.md 😢
• Swift example code is missing the link in the index of README.md

Testing

• Tested on:
  • go version go1.24.1 darwin/amd64
  • go version go1.24.1 linux/amd64 | golang:alpine @ Docker
• This code should work in Go v17+.

Reviewer Nudging

Local run

If Go (golang, v1.17+) is installed locally:

1. Create a main.go file in the working directory
2. Copy & paste the example code of Go to main.go
3. Replace the token and secret variable in the main.go
4. Initialize the modules and build/run the code:

$ go mod init example/switchbot && go mod tidy
**snip**

$ go run main.go
**snip**

Docker run

Using docker may be useful if Go/golang is not installed locally:

1. Create a main.go file in the working directory
2. Copy & paste the example code of Go to main.go
3. Replace the token and secret variable in the main.go
4. Pull the latest Go image of Docker

   $ docker pull golang:alpine

5. Mount the source code and instantiate a container and run:

   $ docker run --rm -it -v "$(pwd)":/app -w /app golang:alpine /bin/sh
   /app # go mod init example/switchbot && go mod tidy
   **snip**
   
   /app # go run main.go
   **snip**