message: add doc test for matrix market save function
diff: @@ -198,7 +198,18 @@ where I: SpIndex,
  /// Write a sparse matrix into the matrix market format.
  ///
  -/// TODO: add example once it's possible to save a compressed matrix
  +/// # Example
  +///
  +/// ```rust,no_run
  +/// use sprs::{CsMat};
  +/// # use std::io;
  +/// # fn save_id5() -> Result<(), io::Error> {
  +/// let save_path = "/tmp/identity5.mm";
  +/// let eye : CsMat<f64> = CsMat::eye(5);
  +/// sprs::io::write_matrix_market(&save_path, &eye)?;
  +/// # Ok(())
  +/// # }
  +/// ```
  pub fn write_matrix_market<'a, N, I, M, P>(path: P, mat: M)
  -> Result<(), io::Error>
  where I: 'a + SpIndex + fmt::Display,
  