Update CHANGELOG.md

Author: gencer

CHANGELOG.md

@@ -6,6 +6,7 @@
 * [#1688](https://github.com/ruby-grape/grape/pull/1688): Removes yard docs - [@ramkumar-kr](https://github.com/ramkumar-kr).
 * [#1702](https://github.com/ruby-grape/grape/pull/1702): Added danger-toc, verify correct TOC in README - [@dblock](https://github.com/dblock).
 * [#1711](https://github.com/ruby-grape/grape/pull/1711): Automatically coerce arrays and sets of types that implement a `parse` method - [@dslh](https://github.com/dslh).
+* [#1720](https://github.com/ruby-grape/grape/pull/1720): Added note for avoid using :default on exactly_one_of - [@gencer](https://github.com/gencer).
 
 #### Fixes