Add the instruction how to update the explorer when the new genesis is presented

[youngjoon-lee]

If Panacea is upgraded with the new genesis (hard-forking) as below (for example):
https://github.com/medibloc/panacea-launch/blob/master/panacea-2/upgrade-validators.md

the explorer must be restarted with the clean DB, because the chain will be started from the height 0.

The important thing is that the old DB must be backed up for preserving old history (such as transactions, and so on).

We already did it when upgrading Panacea from v1.2.5 to v1.3.3, but the official instruction must be written on README or somewhere else.