Add rounding docs.

docs/plugins/round.md

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+# round
+
+Rounds numbers to a shorter form.
+
+## Options
+
+- `coordDigits`: **number** - the number of digits after the decimal place to round x/y coordinates and width/height values (default: **4**)
+- `opacityDigits`: **number** - the number of digits after the decimal place to round opacity values (default: **3**)
+
+## Details
+
+The plugin has no effect if:
+
+- The document has scripts.
+- The document has `<style>` elements with CSS which **svgo-ll** is not able to process.
+- The document has `<style>` elements with attribute selectors.
+
+### x and y coordinates and width/height
+
+Coordinate values and lengths are rounded according to the context in which they are used. The number of digits to which a value is rounded is based on the `viewBox` context in which it is used (or if there is no `viewBox`, the `width` and `height`). The number of digits specified by the `coordDigits` parameter specifies the number of digits for a dimension >= 100 pixels and < 1000 pixels. Smaller dimensions will add one digit for each factor of 10, and larger dimensions will subtract one digit for each factor of 10.
+
+The following values are rounded using this value:
+
+- `M`, `m`, `L`, `l`, `H`, `h`, `V`, or `v` command values in the `d` attribute of a path element.
+- The coordinates in a `translate()` function in a `transform` attribute.
+- The `x`, `y`, `width` and `height` attributes of a `<rect>` element.
+- The `x1`, `y1`, `x2` and `y2` attributes of a `<line>` element.
+
+If the dimension of the element or its ancestors cannot be determined, or if the element or one of its ancestors is subject to a `transform` other than `translate()`, no rounding is done.
+
+### Opacities
+
+`opacity` and `fill-opacity` values are rounded to the number of digits specified by the `opacityDigits` parameter.
+
+### Colors
+
+Any colors specified using RGB percentages are converted to integer RGB values, and minified as they are in the [minifyColors](./minifyColors.md) plugin.