Mistake in installation.md

Thank you for this project!

In installation.md, this is incorrect:

```
export default [
  ...svelteTailwindcss['flat/base']
];
```

it should be:

```
export default [
  ...svelteTailwindcss.configs['flat/base']
];
```

Also, in the main README.md, you might want to place a reference to installation.md, as right now it takes some searching to find the installation info. :-)