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

Author: zpg6

README.md

@@ -37,6 +37,18 @@ while !my_file.is_eof() {
 }
 ```
 
+For writing files:
+```rust
+let my_other_file = root_dir.open_file_in_dir("MY_OTHER_FILE.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()?;
+```
+
 ### Open directories and files
 
 By default the `VolumeManager` will initialize with a maximum number of `4` open directories, files and volumes. This can be customized by specifying the `MAX_DIR`, `MAX_FILES` and `MAX_VOLUMES` generic consts of the `VolumeManager`: