doc(test): document how to replay a block.

docs/test/replay-block.md

+# How to replay a block
+
+You can use `try-runtime` subcommand to replay a block against a real satte from a live network.
+
+1. Checkout the git tag of the runtime version at the block you want to replay
+2. Build duniter with feature `try-runtime`: `cargo build --features try-runtime`
+3. Find the hash of the block to replay
+4. Choose an RPC endpoint without path (try-runtime not support path)
+5. Replay the block a first time to get the state:
+
+```
+duniter try-runtime --exectuion=Native execute-block --block-at 0x2633026e3e428b010cfe08d215b6253843a9fe54db28748ca56de37e6a83c644 live -s tmp/snapshot1 -u ws://localhost:9944
+```
+
+6. Then, replay the block as many times as you need against your local snapshot:
+
+```
+duniter try-runtime --exectuion=Native execute-block --block-at 0x2633026e3e428b010cfe08d215b6253843a9fe54db28748ca56de37e6a83c644 --block-ws-uri ws://localhost:9944 snap -s tmp/snapshot1
+```
+
+try-runtime does not allow (for now) to store the block locally, only the storage can be stored.