[Docs] Mention that in mangle.json, the mangle object should be set under "minify" object

[kenrick95]

In both Readme and mangle.json wiki page, it mentions about how to mangle properties using mangle.json file.

It said:

This can be done in a mangle.json configuration file, [.....] follow the same object format:

{
  "mangle": {
    "properties": {
      "regex": "^_"
    }
  }
}

I assumed that mangle.json can contain exactly that, but apparently, after reading microbundle's source code, mangle.json expects that the configuration to be parked under "minify". So mangle.json should look something like this:

{
  "minify": {
    "mangle": {
      "properties": {
        "regex": "^_"
      }
    }
  }
}

Looks like this behavior is not documented anywhere 😅

[developit]

~Hmm - this shouldn't be required, mangle.json should support "properties" as a top-level key. I don't believe it is nested under a mangle property, perhaps that's the issue?~

Ah, I stand corrected! This must have changed in 0.13. I think it might be worth making that key optional - this file is unlikely to ever contain any other configuration than mangling.