Link to new contributing documentation (#578)

We moved the contributing documentation out of the problem specifications repository.

Test suites:
exercism/docs#4
exercism/docs#15
exercism/docs#66

Anatomy:
exercism/docs#74

Author: Katrina Owen

README.md

@@ -54,11 +54,11 @@ Of course you can also add a totally new exercise, but it might be a good idea t
 
 #### Updating an exercise test suite
 Updating a test suite of an existing exercise is special because it usually affects all languages.
-You can refer to the [documentation on updating a test suite](https://github.com/exercism/x-common/blob/master/CONTRIBUTING.md#updating-an-exercise-test-suite).
+You can refer to the [documentation on updating a test suite](https://github.com/exercism/docs/blob/master/contributing-to-language-tracks/README.md#changing-exercise-test-suites) and the [test suite principles](https://github.com/exercism/docs/blob/master/language-tracks/exercises/anatomy/test-suites.md).
 Note that the whole test suite must run with the sample solution within a couple of seconds.
 
 ### Repository structure and conventions
-The [track anatomy documentation](https://github.com/exercism/x-common/blob/master/CONTRIBUTING.md#track-anatomy) is a general description of all the files and directories that are not explicitly described below.
+The [track anatomy documentation](https://github.com/exercism/docs/blob/master/language-tracks/README.md) is a general description of all the files and directories that are not explicitly described below.
 
 #### Directory structure
 ```bash