Update CHANGELOG.md

Author: radeksimko

CHANGELOG.md

@@ -9,6 +9,7 @@ ENHANCEMENTS:
  - Add support for state backends ([#544](https://github.com/hashicorp/terraform-ls/pull/544))
  - Add support for provisioners ([#542](https://github.com/hashicorp/terraform-ls/pull/542))
  - Support for type declarations (for variable `type`) ([#490](https://github.com/hashicorp/terraform-ls/pull/490))
+ - Support variable `default` completion, hover & semantic highlighting ([#543](https://github.com/hashicorp/terraform-ls/pull/543))
 
 ## 0.17.1 (26 May 2021)