Update api documentation to show commonjs option
Update api documentation to show commonjs option

The change from v1 to v2 introduces ES modules, which is still experimental in Node.js, and is only supported without a flag in Node.js 13, so not widely used in Node yet.

For those upgrading to v2 of PurgeCSS from v1 using CommonJS, they need to either destructure the { PurgeCSS } class from the require statement or use `.default` after the require to import the default export of the PurgeCSS class.

in Purge CSS v1 this used to work

```
const PurgeCSS = require('purgecss')
```

for it to work with CommonJS in v2 it needs to be changed to this

```
const { PurgeCSS } = require('purgecss')
```

or this

```
const PurgeCSS = require('purgecss').default
```

if you don't make this change from v1 to v2 you'll get a 'TypeError: PurgeCSS is not a constructor` error, as the PurgeCSS class is not properly imported
FullHuman/purgecss,FullHuman/purgecss,FullHuman/purgecss,FullHuman/purgecss