Add documentation on how to integrate Linaria

## Describe the enhancement

Firstly we add more chapters to `docs/CONFIGURATION.md` to group working integrations
- [ ] Next.js heading and link to [`with-linaria` example](https://github.com/zeit/next.js/tree/master/examples/with-linaria)
- [x] Gatsby.js heading with a link to [gatsby-plugin-linaria](https://github.com/silvenon/gatsby-plugin-linaria) and optional custom config. Custom config can base on this [gist](https://gist.github.com/jayu/0436ca3dc920470c9d8208f22d6614f9)
- [ ] CRA craco plugin https://github.com/jedmao/craco-linaria
- [x] Preact - based on config posted in https://github.com/callstack/linaria/issues/434#issuecomment-596817660
- [x] Svelte - based on configs posted in https://github.com/callstack/linaria/issues/435#issuecomment-611226593 (**in progress**)

Then modify [Setup section in README](README.md#setup) and list there all possible working integrations from `docs/CONFIGURATION.md` with links obviously.

I'm not sure what to do with Linaria [parcel plugin](https://github.com/callstack/parcel-plugin-linaria). I mean we can link it anyway, but I don't know if it is stable.

## Motivation

There are tool/frameworks that work with linaria but integrations are not documented

## Possible implementations

## Related Issues

#434 , #435