Adds instructions for enabling cloud sync

[EricMountain]

I know Cloud Sync is out of scope for the project. However would it be OK to merge some instructions to enable it for folks that want local file encryption in case the device is lost/stolen, but are willing to accept the reMarkable Cloud attack vector?

[rtpt-erikgeiser]

Thanks for the PR. We can definitely point users in the right direction if they want to enable cloud sync as long as we add the necessary disclaimers. We'll think about how exactly we want to present this information in this repository and merge it in one way or another as soon as we find the time.