Add markdown file documenting css.js features

css.md

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+# CSS
+
+A simple module for generating CSS from JS and a helper for custom properties
+aka. CSS variables.
+
+## Interfaces
+
+```js
+import {css} from 'css.js'
+
+// CamelCase will be turned into kebab-case
+css({div: {backgroundColor: "red"}})
+// will produce:
+// div {background-color: red}
+
+css({div: {
+	transition: ["border 1s", ["color", "2s"]]
+}})
+// will produce:
+// div {transition: border 1s, color 12}
+
+// Nesting will be resolved
+css({
+	".foo": {
+		color: "purple",
+		".bar": {
+			color: "red",
+		},
+		".baz": {
+			color: "blue",
+		},
+	},
+})
+// will produce:
+// .foo {color: purple}
+// .foo .bar {color: red}
+// .foo .baz {color: blue}
+
+// Nested lists will be resolved too
+css({
+	".foo": {
+		color: "purple",
+		".bar, .baz": {
+			color: "red",
+		},
+	},
+})
+// will produce:
+// .foo {color: purple}
+// .foo .bar, .foo .baz {color: red}
+
+// A leading _ will remove the space when nesting
+css({
+	".foo": {
+		color: "black",
+		"_bar": { color: "red" }
+	}
+})
+// will produce:
+// .foo {color: black}
+// .foobar {color: red}
+
+// $ will be replaced with . for convenience
+// This can be combined with _ for to avoid quoting keys
+css({
+	$foo: {
+		_$bar$baz: {
+			color: 'red'
+		}
+	}
+})
+// will produce:
+// .foo.bar.baz {color: red}
+```
+
+Variables helper:
+
+```js
+import {variable as v} from 'css.js'
+
+// Index for automatic hyphenation
+v.fooBar.toString() // var(--foo-bar)
+v.fooBar("Hello") // var(--foo-bar, Hello)
+
+// Call for unchanged variable names
+v("fooBar") // var(--fooBar)
+v("fooBar", "Hello") // var(--fooBar, Hello)
+```
+
+## Examples