Add docs

README.md

@@ -1,6 +1,23 @@
 # subset-font
 
-Create a subset font from an existing font in SFNT (TrueType/OpenType), WOFF, or WOFF2 format. Uses [`harfbuzzjs`](https://github.com/harfbuzz/harfbuzzjs), which is a WebAssembly build of [HarfBuzz](https://harfbuzz.github.io/).
+Create a subset font from an existing font in SFNT (TrueType/OpenType), WOFF, or WOFF2 format. When subsetting a variable font, you can also reduce the variation space at the individual axis level.
+
+These operations are implemented using [`harfbuzzjs`](https://github.com/harfbuzz/harfbuzzjs), which is a WebAssembly build of [HarfBuzz](https://harfbuzz.github.io/).
+
+## Basic example
+
+```js
+const subsetFont = require('subset-font');
+
+const mySfntFontBuffer = Buffer.from(/*...*/);
+
+// Create a new font with only the characters required to render "Hello, world!" in WOFF2 format:
+const subsetBuffer = await subsetFont(mySfntFontBuffer, 'Hello, world!', {
+  targetFormat: 'woff2',
+});
+```
+
+## Reducing the variation space
 
 ```js
 const subsetFont = require('subset-font');
@@ -10,6 +27,15 @@ const mySfntFontBuffer = Buffer.from(/*...*/);
 // Create a new font with only the characters required to render "Hello, world!" in WOFF2 format:
 const subsetBuffer = await subsetFont(mySfntFontBuffer, 'Hello, world!', {
   targetFormat: 'woff2',
+  variationAxes: {
+    // Pin the axis to 200:
+    wght: 200,
+    // Reduce the variation space, explicitly setting a new default value:
+    GRAD: { min: -50, max: 50, default: 25 },
+    // Reduce the variation space. A new default value will be inferred by clamping the old default to the new range:
+    slnt: { min: -9, max: 0 },
+    // The remaining axes will be kept as-is
+  },
 });
 ```
 
@@ -25,6 +51,7 @@ Options:
 
 - `targetFormat` - the format to output, can be either `'sfnt'`, `'woff'`, or `'woff2'`.
 - `preserveNameIds` - an array of numbers specifying the extra name ids to preserve in the `name` table. By default the harfbuzz subsetter drops most of these. Use case described [here](https://github.com/papandreou/subset-font/issues/7).
+- `variationAxes` - an object specifying a full or partial instancing of variation axes in the font. Only works with variable fonts. See the example above.
 
 For backwards compatibility reasons, `'truetype'` is supported as an alias for `'sfnt'`.