Add v3.20.0 to CHANGELOG [ci skip]
Add v3.20.0 to CHANGELOG
[ci skip]

(cherry picked from commit 13ec04419c77371a4ca020978860fb63c684b458)

stefanpenner/ember.js,stefanpenner/ember.js,givanse/ember.js,knownasilya/ember.js,givanse/ember.js,sly7-7/ember.js,givanse/ember.js,sandstrom/ember.js,sandstrom/ember.js,asakusuma/ember.js,asakusuma/ember.js,mfeckie/ember.js,elwayman02/ember.js,mfeckie/ember.js,emberjs/ember.js,givanse/ember.js,mixonic/ember.js,asakusuma/ember.js,knownasilya/ember.js,mixonic/ember.js,elwayman02/ember.js,sly7-7/ember.js,elwayman02/ember.js,knownasilya/ember.js,emberjs/ember.js,tildeio/ember.js,sandstrom/ember.js,stefanpenner/ember.js,mfeckie/ember.js,elwayman02/ember.js,mixonic/ember.js,tildeio/ember.js,tildeio/ember.js,mfeckie/ember.js,asakusuma/ember.js,sly7-7/ember.js,emberjs/ember.js