added a bit of a readme

Author: geelen

README.md

@@ -1,2 +1,51 @@
-# postcss-modules-constants
+# CSS Modules: Constants
+
 Pass arbitrary constants between your module files
+
+### Usage
+
+```css
+/* colors.css */
+@value primary: #BF4040;
+@value secondary: #1F4F7F;
+
+.text-primary {
+  color: primary;
+}
+
+.text-secondary {
+  color: secondary;
+}
+```
+
+```css
+/* breakpoints.css */
+@value small: (max-width: 599px);
+@value medium: (min-width: 600px) and (max-width: 959px);
+@value large: (min-width: 960px);
+```
+
+```css
+/* my-component.css */
+@value colors: "./colors.css";
+@value primary, secondary from colors;
+@value small as bp-small, large as bp-large from "./breakpoints.css";
+
+.header {
+  composes: text-primary from colors;
+  box-shadow: 0 0 10px secondary;
+}
+
+@media bp-small {
+  .header {
+    box-shadow: 0 0 4px secondary;
+  }
+}
+@media bp-large {
+  .header {
+    box-shadow: 0 0 20px secondary;
+  }
+}
+```
+
+Note that the `:` in a `@value` definition is optional as to not break Sass.