Add 0.6.21 into CHANGELOG.md
Add 0.6.21 into CHANGELOG.md

vhtien/gooddata-ruby,vhtien/gooddata-ruby,lubosvesely/gooddata-ruby,Seikitsu/gooddata-ruby,lubosvesely/gooddata-ruby,vhtien/gooddata-ruby,lubosvesely/gooddata-ruby,Seikitsu/gooddata-ruby,Seikitsu/gooddata-ruby