Updated contribution guidelines to explicitely state that any code contribution should include new tests.

Author: donigian

CONTRIBUTING.md

@@ -114,7 +114,8 @@ following rules before you submit a pull request:
    to other methods available in scikit-learn.
 
 -  Documentation and high-coverage tests are necessary for enhancements to be
-   accepted. Bug-fixes or new features should be provided with 
+   accepted. We encourage you to add new test(s) as it's relatively rare for any code 
+   contributions not to require test(s). Bug-fixes or new features should be provided with 
    [non-regression tests](https://en.wikipedia.org/wiki/Non-regression_testing).
    These tests verify the correct behavior of the fix or feature. In this
    manner, further modifications on the code base are granted to be consistent

doc/developers/contributing.rst

@@ -825,7 +825,8 @@ each pull request needs to be signed off by two core developers, you can speed
 up this process by providing your feedback.
 
 Here are a few important aspects that need to be covered in any code review,
-from high-level questions to a more detailed check-list.
+from high-level questions to a more detailed check-list. We encourage you to add new 
+test(s) as it's relatively rare for any code contributions not to require test(s).
 
 - Do we want this in the library? Is it likely to be used? Do you, as
   a scikit-learn user, like the change and intend to use it? Is it in