Added basic guidelines for contributing code (#976)

* Update README.md

* Apply instructions for pull requests from Slack to new CONTRIBUTING guideline file.

I copied the [message in Slack](https://etlcpp.slack.com/archives/C7SJ45VFB/p1729596737002559) from @jwellbelove into a new file for [contributing guidelines](https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors). This way a starting point for potential contributors is delivered next to the source code.

* Added hint for the starting point for contributing commits.

I derived this rule from #802 (comment)

---------

Co-authored-by: John Wellbelove <[email protected]>

CONTRIBUTING.md

@@ -0,0 +1,10 @@
+# How to contribute
+
+If your are considering creating a pull request, please observe the following:
+
+- If you are adding or modifying a feature, add *new* unit tests that test that feature.
+- If you are fixing a bug, add a unit test that *fails* before the bug fix is implemented.
+- Do not initiate a pull request until all of the units tests pass.
+- Branches should be based on the branch `master`.
+
+There is a project file for VS2022 for C++14, 17, 20, and bash scripts that run the tests for C++11, 14, 17, 20 under Linux with GCC and Clang.