docs(log): add an example for using a logfile

members/libpt-log/examples/logfile.rs

@@ -0,0 +1,11 @@
+use libpt_log::Logger;
+use tracing::info;
+
+fn main() {
+    let _logger = Logger::builder()
+        .log_to_file(true)
+        .log_dir("/tmp/llll".into())
+        .build()
+        .unwrap();
+    info!("foo bar qux");
+}