Add limitation file

doc/limitations.md

+# Limitations
+
+The philosophy of mruby is to be a lightweight implementation of
+the Ruby ISO standard. These two objectives are partially contradicting
+due to the reason that Ruby is a powerful language with complex
+implementation details which are difficult to implement in a lightweight
+manner.
+
+This document is collecting these limitations.
+
+## Integrity
+
+This document does not contain a complete list of limitations.
+Please help to improve it by submitting your findings.
+
+## Confirmed limitations
+
+### ```Array``` passed to ```puts```
+
+Passing an Array to ```puts``` results in different output.
+
+```ruby
+puts [1,2,3]
+```
+
+#### Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
+
+```
+1
+2
+3
+```
+
+#### mruby [1.2.0 (2015-11-17)]
+
+```
+[1, 2, 3]
+```
+
+### ```Kernel.raise``` in rescue clause
+
+```Kernel.raise``` without arguments does not raise the current exception within
+a rescue clause.
+
+```ruby
+begin
+  1 / 0
+rescue
+  raise
+end
+```
+
+#### Ruby [ruby 2.0.0p645 (2015-04-13 revision 50299)]
+
+```ZeroDivisionError``` is raised.
+
+#### mruby [1.2.0 (2015-11-17)]
+
+No exception is raised.