Add the HFC cross era ticking caveat to the AddingAnEra document

We should document the caveats that should be taken into account when adding a new era, when it comes to era transition and cross era ticking. The goal is to help us prevent issues [like this one](https://github.com/input-output-hk/cardano-ledger/issues/3491) from occurring. In particular, we should explain the rationale for patches like [this one](https://github.com/input-output-hk/ouroboros-consensus/pull/366).