docs: README write to file example; reminder to run file.flush()

[zpg6]

Adding example of writing to files in the code snippet. Includes a reminder to run file.flush() so that directory entry is updated (I has missed this resulting in #172).

let my_other_file = root_dir.open_file_in_dir("MY_DATA.CSV", embedded_sdmmc::Mode::ReadWriteCreateOrAppend)?;
my_other_file.write(b"Timestamp,Signal,Value\n")?;
my_other_file.write(b"2025-01-01T00:00:00Z,TEMP,25.0\n")?;
my_other_file.write(b"2025-01-01T00:00:01Z,TEMP,25.1\n")?;
my_other_file.write(b"2025-01-01T00:00:02Z,TEMP,25.2\n")?;

// Don't forget to flush the file so that the directory entry is updated
my_other_file.flush()?;

[thejpster]

Thank you for the changes. They look good to me.