Add a section for timestamp

[GopherJ]

zksync timestamp got updated during new batch, a section is needed for describing this to resolve the following issue:

• how developers should handle the timestamp?
• do we have any timestamp contract to use?

[MexicanAce]

@GopherJ Perhaps this link/doc helps provide information regarding the timestamp? https://era.zksync.io/docs/reference/concepts/blocks.html#block-number-and-timestamp-considerations