docs: adds same snippet of writing to files to lib.rs for crate doc

Author: zpg6

src/lib.rs

@@ -43,6 +43,19 @@
 //! }
 //! ```
 //!
+//! For writing files:
+//!
+//! ```rust
+//! 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()?;
+//! ```
+//!
 //! ## Features
 //!
 //! * `log`: Enabled by default. Generates log messages using the `log` crate.