Update README.md

created known issues section

Author: marabesi

README.md

@@ -18,6 +18,12 @@ For example if there is an image subfolder under your extension project workspac
 
 ![Change highlight color](demo-color.gif)
 
+## Known Issues
+
+- The extension does not support traits. ([#3](https://github.com/marabesi/php-import-checker/issues/3))
+- If the imported class with `use` is inside a comment block, the extension is
+not going to highlight as a unused class. ([#10](https://github.com/marabesi/php-import-checker/issues/10))
+
 
 
 ## Requirements