simapp: add testnet instructions

[tessr]

Description

The Tendermint Core team sometimes uses simapp to run small testnets while we're trying out changes to Tendermint. This documents the steps we take to set up these small testnets with simapp.

I'm not totally sure this belongs in the Cosmos SDK repo; if not, we can stick it somewhere in our team's internal documentation instead. But I bet it might be useful to other teams who use simapp for testing!

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