Update README.md

README.md

@@ -1,13 +1,25 @@
 # ember-cli-clean-css
 
-[![Greenkeeper badge](https://badges.greenkeeper.io/ember-cli/ember-cli-clean-css.svg)](https://greenkeeper.io/)
-
 ## Installation
 
 ```
 ember install ember-cli-clean-css
 ```
 
+## Usage
+
+Options for clean-css can be passed through the `minifyCSS` property in `ember-cli-build.js`. Please see the docs for ember-cli [here](https://cli.emberjs.com/release/advanced-use/asset-compilation/#minifying).
+
+## clean-css versions
+
+### `ember-cli-clean-css` 3.x 
+
+`ember-cli-clean-css` 3.x uses `clean-css` `v5.0`. When upgrading you may enounter differences in css output. Please see the `clean-css` [documentation](https://github.com/clean-css/clean-css) and changelog to determine how the output has changed.
+
+### `ember-cli-clean-css` 2.x 
+
+`ember-cli-clean-css` 2.x uses `clean-css` `v3.4`. This exactly matches the version that `ember-cli` (via `ember-cli-preprocess-registry`) used in `ember-cli <= 4.12`. 
+
 ## Contributing
 
 See the [Contributing](CONTRIBUTING.md) guide for details.