update readme

readme.md

@@ -39,11 +39,33 @@ Example use:
         [0.0],
         ];
 
-    let (res_x, err, iters) = gmres(a_test.as_ref(), b.as_ref(), x0.as_ref(), 10, 1e-8).unwrap();
+    // the final None arg means do not apply left preconditioning
+    let (res_x, err, iters) = gmres(a_test.as_ref(), b.as_ref(), x0.as_ref(), 10, 1e-8, None).unwrap();
     println!("Result x: {:?}", res_x);
     println!("Error x: {:?}", err);
     println!("Iters : {:?}", iters);
 
+## Preconditioned GMRES:
+
+A preconditioner can be supplied:
+
+    // continued from above...
+    use faer_gmres::{JacobiPreconLinOp, LinOp};
+    let jacobi_pre = JacobiPreconLinOp::new(a_test.as_ref());
+    let (res_x, err, iters) = gmres(a_test.as_ref(), b.as_ref(), x0.as_ref(), 10, 1e-8, Some(&jacobi_pre)).unwrap();
+
+## Restarted GMRES:
+
+A restarted GMRES routine is provided:
+
+    use faer_gmres::restarted_gmres;
+    let max_inner = 30;
+    let max_outer = 50;
+    let (res_x, err, iters) = restarted_gmres(
+        a_test.as_ref(), b.as_ref(), x0.as_ref(), max_inner, max_outer, 1e-8, None).unwrap();
+
+This will repeatedly call the inner GMRES routine, using the previous outer iteration's solution as the inital guess for the next outer solve.  The current implementation of restarted GMRES in this package can reduce the memory requirements needed, but slow convergence.
+
 TODO
 ====