add readme

README.md

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 # tiny-solver-rs
+Inspired by ceres-solver and tiny-solver. This is a re-implemented version of [tiny-solver](https://github.com/keir/tinysolver/tree/master) in rust.
+
+### Usage
+You need to implement the cost function of `TinySolver<NUM_PARAMETERS, NUM_RESIDUALS>`.
+Just like ceres-solver.
+
+See full example in `examples/example_solve.rs`.
+```rust
+impl TinySolver<3, 2> for ExampleStatic {
+    fn cost_function(
+        params: nalgebra::SVector<num_dual::DualSVec64<3>, 3>,
+    ) -> nalgebra::SVector<num_dual::DualSVec64<3>, 2> {
+        let x = params[0];
+        let y = params[1];
+        let z = params[2];
+        return nalgebra::SVector::from([x + y.mul(2.0) + z.mul(4.0), y * z]);
+    }
+}
+```