message: show usage of TriMat on entry page
diff: @@ -22,8 +22,24 @@ It features a sparse matrix type, [**`CsMat`**](struct.CsMatBase.html), and a sp
  ## Quick Examples
  +
  Matrix construction:
  +```rust
  +use sprs::TriMat;
  +
  +let mut a = TriMat::new((4, 4));
  +a.add_triplet(0, 0, 3.0_f64);
  +a.add_triplet(1, 2, 2.0);
  +a.add_triplet(3, 0, -2.0);
  +
  +// This matrix type does not allow computations, and must to
  +// converted to a compatible sparse type, using for example
  +let b = a.to_csr();
  +```
  +
  +Constructing matrix using the more efficient direct sparse constructor
  +
  ```rust
  use sprs::{CsMat, CsVec};
  let eye : CsMat<f64> = CsMat::eye(3);
  