Add a known issue to README for Ruby bug 12876

See freerange#276 and the Ruby bug report [1] for more details.

[1]: https://bugs.ruby-lang.org/issues/12876

README.md

@@ -110,6 +110,7 @@ require 'mocha/mini_test'
 #### Known Issues
 
 * In Mocha v1.2.0 there is a scenario where stubbing a class method originally defined in a module hangs the Ruby interpreter due to [a bug in Ruby v2.3.1](https://bugs.ruby-lang.org/issues/12832). See #272. This was fixed in Mocha v1.2.1.
+* Since v1.1.0 Mocha has used prepended modules internally for stubbing methods. There is [an obscure Ruby bug](https://bugs.ruby-lang.org/issues/12876) in many (but not all) versions of Ruby between v2.0 & v2.3 which under certain circumstances may cause your Ruby interpreter to hang. See the Ruby bug report for more details. The bug has been fixed in Ruby v2.3.3 & v2.4.0.
 * Stubbing an aliased class method, where the original method is defined in a module that's used to `extend` the class doesn't work in Ruby 1.8.x. See stub_method_defined_on_module_and_aliased_test.rb for an example of this behaviour.
 * 0.13.x versions cause a harmless, but annoying, deprecation warning when used with Rails 3.2.0-3.2.12, 3.1.0-3.1.10 & 3.0.0-3.0.19.
 * 0.11.x versions don't work with Rails 3.2.13 (`TypeError: superclass mismatch for class ExpectationError`). See #115.