README: document how to run obfsvpnintro backend

#!/bin/bash
set -eux

cp server/test_data/obfs4.json obfsvpn.yml
mkdir -p state/

go run ./cmd/server \
    --addr 127.0.0.1 \
    --port 4430 \
    --remote "127.0.0.1:8443" \
    --state $(pwd)/state \
    --persist \
    -v