doc: pimp readme (#12)

README.md

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 # <div align="center"> PowerBoxes </div>
-Powerboxes is a package containing utility functions for transforming bounding boxes and computing metrics from them
+Powerboxes is a package containing utility functions for transforming bounding boxes and computing metrics. It is implemented in both Python and Rust.
+It shows a significant speedup over the equivalent numpy implementations in Python, or other libraries such as [shapely](https://github.com/shapely/shapely).
+
+## Installation
+
+### Python
+```bash
+pip install powerboxes
+```
+
+### Rust
+```bash
+cargo add powerboxesrs
+```
+
+## Python Usage
+
+```python
+import powerboxes as pb
+import numpy as np
+
+# Create a bounding box
+box = np.array([[0, 0, 1, 1]])
+
+# Compute the area of the box
+area = pb.box_areas(box)
+
+# Compute the intersection of the box with itself
+intersection = pb.iou_distance(box, box)
+```
+### Functions available
+#### Box Transformations and utilities
+- `box_areas`: Compute the area of list of boxes
+- `box_convert`: Convert a box from one format to another. Supported formats are `xyxy`, `xywh`, `cxcywh`.
+- `remove_small_boxes`: Remove boxes with area smaller than a threshold
+
+#### Box Metrics
+- `iou_distance`: Compute the intersection over union matrix of two sets of boxes
+- `parallel_iou_distance`: Compute the intersection over union matrix of two sets of boxes in parallel
+- `giou_distance`: Compute the generalized intersection over union matrix of two sets of boxes
+- `parallel_giou_distance`: Compute the generalized intersection over union matrix of two sets of boxes in parallel
+
+
+## Use it in Rust
+See the [documentation](https://docs.rs/powerboxesrs) for more details.
+Here is a simple example:
+```rust
+use ndarray::array;
+use powerboxesrs::boxes::box_areas;
+let boxes = array![[1., 2., 3., 4.], [0., 0., 10., 10.]];
+let areas = box_areas(&boxes);
+assert_eq!(areas, array![9., 121.]);
+```
+