Describe the difference of the keyword argument behavior.

The implementation of keyword arguments is heavily rely on the prototype
made by @take-cheeze in mruby#3629.

doc/limitations.md

@@ -205,3 +205,25 @@ trace (most recent call last):
         [0] -e:1
 -e:1: undefined method 'binding' (NoMethodError)
 ```
+
+## Keyword arguments
+
+mruby keyword arguments behave slightly different from CRuby 2.5
+to make the behavior simpler and less confusing. Maybe in the
+future, the simpler behavior will be adopted to CRuby as well.
+
+#### Ruby [ruby 2.5.1p57 (2018-03-29 revision 63029)]
+
+```
+$ ruby -e 'def m(*r,**k) p [r,k] end; m("a"'=>1,:b=>2)'
+[[{"a"=>1}], {:b=>2}]
+```
+
+#### mruby []
+
+```
+$ ./bin/mruby -e 'def m(*r,**k) p [r,k] end; m("a"'=>1,:b=>2)'
+trace (most recent call last):
+	[0] -e:1
+-e:1: keyword argument hash with non symbol keys (ArgumentError)
+```