[Hacktoberfest] add standby signal

[pollosp]

Hello,

I think using standby.signal make example easy to understand as by default uploaded wals from postgres1 backup are going to be recovered in postgres2

From Postgres doc

27.2.2. Standby Server Operation
A server enters standby mode if a standby.signal file exists in the data directory when the server is started.

In standby mode, the server continuously applies WAL received from the primary server.