Segment Anything readme (#827)

* Add a readme for the segment-anything model.

* Add the original image.

* Clean-up the segment anything cli example.

* Also print the mask id in the outputs.

candle-examples/examples/segment-anything/README.md

@@ -0,0 +1,40 @@
+# candle-segment-anything: Segment-Anything Model
+
+This example is based on Meta AI [Segment-Anything
+Model](https://github.com/facebookresearch/segment-anything). This model
+provides a robust and fast image segmentation pipeline that can be tweaked via
+some prompting (requesting some points to be in the target mask, requesting some
+points to be part of the background so _not_ in the target mask, specifying some
+bounding box).
+
+The default backbone can be replaced by the smaller and faster TinyViT model
+based on [MobileSAM](https://github.com/ChaoningZhang/MobileSAM).
+
+## Running some example.
+
+```bash
+cargo run --example segment-anything --release -- \
+    --image candle-examples/examples/yolo-v8/assets/bike.jpg
+    --use-tiny
+    --point-x 0.4
+    --point-y 0.3
+```
+
+Running this command generates a `sam_merged.jpg` file containing the original
+image with a blue overlay of the selected mask. The red dot represents the prompt
+specified by `--point-x 0.4 --point-y 0.3`, this prompt is assumed to be part
+of the target mask.
+
+The values used for `--point-x` and `--point-y` should be between 0 and 1 and
+are proportional to the image dimension, i.e. use 0.5 for the image center.
+
+![Leading group, Giro d'Italia 2021](../yolo-v8/assets/bike.jpg)
+
+![Leading group, Giro d'Italia 2021](./assets/sam_merged.jpg)
+
+### Command-line flags
+- `--use-tiny`: use the TinyViT based MobileSAM backbone rather than the default
+  one.
+- `--point-x`, `--point-y`: specifies the location of the target point.
+- `--threshold`: sets the threshold value to be part of the mask, a negative
+  value results in a larger mask and can be specified via `--threshold=-1.2`.