docs: add contribution guide

Author: zsquareplusc

CONTRIBUTING.rst

@@ -0,0 +1,46 @@
+==============
+ Contributing
+==============
+
+How Can I Contribute?
+=====================
+
+Reporting Bugs and Suggesting Enhancements
+    The easiest is creating an issue_. You can do that for ideas or problems.
+
+    For bugs, if possible, try to construct a small code example that exhibits
+    the problem. Include version numbers (Python, pySerial,
+    pySerial-asyncio).
+
+Pull Requests
+    Code / test / documentation contributions are of course welcome_!
+
+    Consider that this is a project that aims to run on many platforms and
+    architectures and therefore needs implementations covering all of them, of
+    course there are exceptions and starting on one platform is better than
+    starting on none.
+
+    When providing new features, also think of documentation and tests if
+    possible.
+
+
+Ownership
+=========
+
+- By contributing to this project, you agree that the material is labeled as
+  "copyright by pySerial-team". Names or handles of individual contributors are
+  listed in a separate file: CREDITS.rst_.
+
+- Your contribution may be used in its original or modified form.
+
+- The project maintainer(s) can make decisions in the name of
+  the pySerial-team. Including but not limited to:
+
+  - making releases, adding/removing features, changing the design, etc.
+  - adding or removing project maintainers
+  - license changes, as long as it remains a permissive open source license.
+
+
+.. _CREDITS.rst: CREDITS.rst
+.. _issue: https://github.com/pyserial/pyserial-asyncio/issues
+.. _wellcome: https://github.com/pyserial/pyserial-asyncio/pulls