Update README.md

[IAmScRay]

Since docker logs bitcoin-signet-client shows only setup messages like below, many people wonder how to check Signet node sync status

install_done file not found, running install.sh.
Generate or import keyset
Imported signetchallange and privkey being used.
PRIVKEY=
SIGNETCHALLENGE= 512102653734c749d5f7227d9576b3305574fd3b0efdeaa64f3d500f121bf235f0a43151ae
Generate bitcoind configuration
Setup Signet
Bitcoin Core starting
{
  "name": "citrea",
  "warning": "Wallet created successfully. The legacy wallet type is being deprecated and support for creating and opening legacy wallets will be removed in the future."
}
Infinate loop
Error: Cannot obtain a lock on data directory /root/.bitcoin/signet. Bitcoin Core is probably already running.
get magic

This pull request adds some clarity to README documentation. No other documentation was modified.