Update README

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@@ -3,14 +3,64 @@
 This is a little Python script I wrote to generate 3-axis toolpaths to
 engrave raster images.
 
-## Getting Started
+## Dependencies
 
-You just need Python 3, OpenCV, and NumPy (i.e. `pip install ...`).
+You just need Python 3, OpenCV, tqdm, and NumPy (i.e. `pip install
+...`).
 
-Then, just run `python src/rastercarve.py IMAGE`, where `IMAGE` is a
-bitmap image in any format supported by OpenCV. G-code is output to
-standard output.
+## Usage
 
-## Configuration
+The program should be self-documenting. Run `src/rastercarve.py
+--help` or see the output below.
 
-Edit `rastercarve.py` to change the material and engraving settings.
+```
+usage: rastercarve.py [-h] (--width WIDTH | --height HEIGHT) [-f FEED_RATE] [-p PLUNGE_RATE] [--rapid RAPID_RATE] [-z SAFE_Z]
+                      [--end-z TRAVERSE_Z] [-d MAX_DEPTH] [-t TOOL_ANGLE] [-a LINE_ANGLE] [-s STEPOVER] [-r LINEAR_RESOLUTION]
+                      [--no-line-numbers] [--debug] [-q] [--version]
+                      filename
+
+Generate G-code to engrave raster images.
+
+positional arguments:
+  filename              input image (any OpenCV-supported format)
+
+optional arguments:
+  -h, --help            show this help message and exit
+  --debug               print debug messages
+  -q                    disable progress and statistics
+  --version             show program's version number and exit
+
+output dimensions:
+  Exactly one required.
+
+  --width WIDTH         output width (in)
+  --height HEIGHT       output height (in)
+
+machine configuration:
+  -f FEED_RATE          engraving feed rate (in/min) (default: 100)
+  -p PLUNGE_RATE        engraving plunge rate (in/min) (default: 30)
+  --rapid RAPID_RATE    rapid traverse rate (for time estimation only) (default: 240)
+  -z SAFE_Z             rapid Z traverse height (in) (default: 0.1)
+  --end-z TRAVERSE_Z    Z height of final traverse (in) (default: 2)
+  -d MAX_DEPTH          maximum engraving depth (in) (default: 0.08)
+  -t TOOL_ANGLE         included angle of tool (deg) (default: 30)
+
+engraving parameters:
+  -a LINE_ANGLE         angle of grooves from horizontal (deg) (default: 22.5)
+  -s STEPOVER           stepover percentage (affects spacing between lines) (default: 110)
+  -r LINEAR_RESOLUTION  distance between successive G-code points (in) (default: 0.01)
+
+G-code parameters:
+  --no-line-numbers     suppress G-code line numbers (dangerous on ShopBot!)
+
+Defaults are usually safe to leave unchanged.
+```
+
+### Examples
+
+```
+python src/rastercarve.py --width 10 examples/test.png > out.nc
+```
+
+Generate G-code to engrave `examples/test.png` into an image 10 inches
+wide. Output will be piped from stdout to `out.nc`.