[SYCL][Doc] Added requirement on reference to test PR in commit message (#3010)

Author: vladimirlaz

CONTRIBUTING.md

@@ -62,6 +62,14 @@ see [ABI Policy Guide](sycl/doc/ABIPolicyGuide.md) for more information.
 - For any DPC++-related commit, the `[SYCL]` tag should be present in the
   commit message title. To a reasonable extent, additional tags can be used
   to signify the component changed, e.g.: `[PI]`, `[CUDA]`, `[Doc]`.
+- For product changes which require modification in tests outside of the current repository
+  (see [Test DPC++ toolchain](sycl/doc/GetStartedGuide.md#test-dpc-toolchain)),
+  the commit message should contain the link to corresponding test PR, e.g.: E2E
+  test changes are available under intel/llvm-test-suite#88 or SYCL
+  conformance test changes are available under KhronosGroup/SYCL-CTS#65 (see
+  [Autolinked references and URLs](https://docs.github.com/en/free-pro-team/github/writing-on-github/autolinked-references-and-urls)
+  for more details). The same message should be present both in commit
+  message and in PR description.
 
 ### Review and acceptance testing