feat(docs) Add description to check if synchronization is complete

[cumaaaaa]

reason

Knowing that registration is not possible until synchronization is completed, it is better to have it in the document to facilitate the registration of validators and reduce the communication of the same.

change

Added explanation that synchronization must be completed before validator registration and how to check synchronization status.

[cumaaaaa]

I couldn't find a repository for the documentation, so if you'd prefer to add it there, let me know and I'll make a PR

If don't need it, let me know and I'll close it