Added removal of to_int and to_str to Breaking Changes.

ref mruby/mruby#4177

Author: matz

_posts/2018-12-10-mruby-2.0.0-released.markdown

@@ -99,6 +99,9 @@ class_variables, class_variables_defined?, class_variable_get, class_variable_se
 - Module class (Singleton Methods)  
 constants, nesting
 
+### 3. No implicit conversion by `to_int` and `to_str`.
+
+The objects with methods `to_int` or `to_str` are no longer converted to Integers or Strings respectively. Those implicit conversions cause recursive VM calls and had been the source of many bugs.
 ---
 
 We have done 378 commits to 156 files, 7,949 lines were added, 5,430 lines removed since mruby1.4.1.
@@ -107,5 +110,3 @@ For more detail of the updates, see [Commit Log](https://github.com/mruby/mruby/
 ## Evaluated mrbgems list (NPO mruby forum)
 
 [See the list](http://forum.mruby.org/download/index2EN.html).
-
-