simapp: add testnet instructions (#8342)

simapp/README.md

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+---
+order: false
+---
+
+# simapp
+
+simapp is an application built using the Cosmos SDK for testing and educational purposes. 
+
+## Running testnets with `simd` 
+
+If you want to spin up a quick testnet with your friends, you can follow these steps. 
+Unless otherwise noted, every step must be done by everyone who wants to participate
+in this testnet.
+
+1. `$ make build`. This will build the `simd` binary and install it in your Cosmos SDK repo, 
+    inside a new `build` directory. The following instructions are run from inside 
+    that directory. 
+2. If you've run `simd` before, you may need to reset your database before starting a new
+    testnet: `$ ./simd unsafe-reset-all` 
+3. `$ ./simd init [moniker]`. This will initialize a new working directory, by default at 
+    `~/.simapp`. You need a provide a "moniker," but it doesn't matter what it is.
+4. `$ ./simd keys add [key_name]`. This will create a new key, with a name of your choosing. 
+    Save the output of this command somewhere; you'll need the address generated here later.
+5. `$ ./simd add-genesis-account  $(simd keys show [key_name] -a) [amount]`, where `key_name`
+    is the same key name as before; and `amount` is something like `10000000000000000000000000stake`.
+6. `$ ./simd gentx [key_name] [amount] --chain-id [chain-id]`. This will create the
+    genesis transaction for your new chain. 
+7. Now, one person needs to create the genesis file `genesis.json` using the genesis transactions 
+   from every participant, by gathering all the genesis transactions under `config/gentx` and then
+   calling `./simd collect-gentxs`. This will create a new `genesis.json` file that includes data
+   from all the validators (we sometimes call it the "super genesis file" to distinguish it from
+   single-validator genesis files). 
+8. Once you've received the super genesis file, overwrite your original `genesis.json` file with 
+    the new super `genesis.json`. 
+9. Modify your `config/config.toml` (in the simapp working directory) to include the other participants as
+    persistent peers:
+
+    ```
+    # Comma separated list of nodes to keep persistent connections to
+    persistent_peers = "[validator address]@[ip address]:[port],[validator address]@[ip address]:[port]"
+    ```
+
+    You can find `validator address` by running `./simd tendermint show-node-id`. (It will be hex-encoded.)
+    By default, `port` is 26656.
+10. Now you can start your nodes: `$ ./simd start`. 
+
+Now you have a small testnet that you can use to try out changes to the Cosmos SDK or Tendermint! 
+
+NOTE: Sometimes creating the network through the `collect-gentxs` will fail, and validators will start
+in a funny state (and then panic). If this happens, you can try to create and start the network first
+with a single validator and then add additional validators using a `create-validator` transaction.