[TM-395] private chain and nettest

Description

Added instructions for running a local chain with short block periods. Such a chain can be used for nettests, when one needs to quickly test some contract against the reference implementation.

Kinda depends on https://github.com/serokell/tezos-packaging/pull/39

Related issue(s)

https://issues.serokell.io/issue/TM-395

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