Add pre-commit instructions to CONTRIBUTING.md (#1606)

Signed-off-by: hiaselhans <[email protected]>

Author: hiaselhans

CONTRIBUTING.md

@@ -77,12 +77,23 @@ for more information.
 - Create a personal fork of the project on GitHub
   - For the DPC++ Compiler project, use **sycl** branch as baseline for your
     changes. See [Get Started Guide](sycl/doc/GetStartedGuide.md).
-- Prepare your patch (follow
-  [LLVM coding standards](https://llvm.org/docs/CodingStandards.html)).
-  [clang-format](https://clang.llvm.org/docs/ClangFormat.html) and [clang-tidy](
-  https://clang.llvm.org/extra/clang-tidy/) tools can be integrated into your
-  workflow to ensure formatting and stylistic compliance of your changes.
+- Prepare your patch
+    - follow [LLVM coding standards](https://llvm.org/docs/CodingStandards.html)
+    - [clang-format](https://clang.llvm.org/docs/ClangFormat.html) and 
+      [clang-tidy](https://clang.llvm.org/extra/clang-tidy/) tools can be integrated into your
+      workflow to ensure formatting and stylistic compliance of your changes.
+    - use
+      ```
+      ./clang/tools/clang-format/git-clang-format `git merge-base origin/sycl HEAD`
+      ```
+      to check the format of your current changes against the `origin/sycl` branch.
+        - `-f` to also correct unstaged changes
+        - `--diff` to only print the diff without applying
 - Build the project and run all tests.
+    - complete test suite: `python buildbot/check.py`
+    - sycl test suite `python buildbot/check.py -t test-sycl`
+    - run only "mytest" test `python buildbot/check.py -t test-sycl-mytest`
+    - if necessary, use `-o $LLVM_BUILD_DIR` to specify the llvm build directory
 
 ### Commit message