feat: Add a note about cookies in documentation#205

### Describe the problem I suggest adding a word about cookies. This is not per say a svelte-adapter-firebase issue, but it took me days of investigating why my cookies were working locally, but not live. The solution was fairly simple, but buried deep inside Firebase's documentation. ### Describe the proposed solution Write a short paragraph under Caveats about the single cookie name that's allowed to be passed to your SSR function, and it is: `__session` It is documented here: [https://firebase.google.com/docs/hosting/manage-cache#using_cookies](https://firebase.google.com/docs/hosting/manage-cache#using_cookies) Copy suggestion: > **Cookies**: When using Firebase Hosting together with Cloud Functions, cookies are stripped from incoming requests. Only the specially-named `__session` cookie is permitted to pass through to the execution of your app. Source: [Firebase documentation](https://firebase.google.com/docs/hosting/manage-cache#using_cookies) ### Alternatives considered One even greater alternative would be to trigger a warning in the console when trying to access an other cookie from the server. I managed to get some cookies across in some occasions, not sure if it's worth implementing. ### Importance would make my life easier