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

In both [Readme](https://github.com/developit/microbundle/blob/master/README.md) and [mangle.json wiki page](https://github.com/developit/microbundle/wiki/mangle.json), 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 😅