[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 😅

Start with the README and the mangle.json wiki page linked in the issue, then compare their current examples with the behavior described from Microbundle's source. Update both documentation locations to show the mangle configuration nested under "minify", and confirm the examples consistently describe the expected mangle.json structure.

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