Add code formatting instructions to CONTRIBUTING.md (#1343)

Also merged testing into main steps

Author: JackPGreen

.github/CONTRIBUTING.md

@@ -1,11 +1,14 @@
 #### Contributing
 If you would like to contribute to hazelcast-cpp-client;
 - Fork the original repo
+- Test your changes - the script executes builds and runs the unit tests for all bit and library versions:
+  - Linux/Mac - [`scripts/test-unix.sh`](../scripts/test-unix.sh)
+  - Windows - [`scripts/test-windows.bat`](../scripts/test-windows.sh)
+- Format the code using `clang-format`
+   - [`scripts/format-all.sh`](../scripts/format-all.sh) can do this for you
 - Commit your contributions in a new branch
 - Send a Pull Request
 
 For detailed instructions for our git process: [Developing with Git](https://hazelcast.atlassian.net/wiki/display/COM/Developing+with+Git)
 
 In order to merge your pull request, we need the contributor to sign [Hazelcast Contributor Agreement](https://hazelcast.atlassian.net/wiki/display/COM/Hazelcast+Contributor+Agreement). You do not have to wait for approval for doing this step.
-
-Please make sure that your code passes `testLinux.sh` for Linux/Mac and `testWindows.bat` for MS Windows. The script executes builds and runs the unit tests for all bit and library versions. (Alternatively, you can also use `testLinuxSingleCase.sh` or `testWindowsSingleCase.sh` for testing with a specific bit aversion and library type.