Add development help

[MahdiBM]

There are no explanations on how to help developing Swiftly.

I personally had to:

1. Use SSWG's VSCode extension and set up a dev container to develop Swiftly on macOS, because otherwise there are some macOS-related errors, and also Swiftly doesn't yet support macOS.
2. Create a file at /root/.local/share/swiftly/config.json containing the json below, because otherwise swiftly would complain about the file and wouldn't work:

{
  "platform" : {
    "nameFull" : "ubuntu22.04",
    "name" : "ubuntu2204",
    "namePretty" : "Ubuntu 22.04.1 LTS"
  },
  "inUse" : null,
  "installedToolchains" : []
}

3. Use swift run swiftly [subcommand].