Add contributing guidelines (#341)

Author: maciejwalkowiak

CONTRIBUTING.md

@@ -0,0 +1,37 @@
+## How to contribute to Spring Cloud AWS
+
+#### **Did you find a bug?**
+
+* **Do not open up a GitHub issue if the bug is a security vulnerability
+  in Spring Cloud AWS**, and instead to refer to our [security policy](https://github.com/awspring/spring-cloud-aws/blob/main/SECURITY.md).
+
+* **Ensure the bug was not already reported** by searching on GitHub under [Issues](https://github.com/awspring/spring-cloud-aws/issues).
+
+* If you're unable to find an open issue addressing the problem, [open a new one](https://github.com/awspring/spring-cloud-aws/issues/new). Be sure to include a **title and clear description**, as much relevant information as possible, ideally with a **code sample** or an **executable test case** demonstrating the expected behavior that is not occurring.
+
+#### **Did you write a patch that fixes a bug?**
+
+* Open a new GitHub pull request with the patch.
+
+* Ensure the PR description clearly describes the problem and solution. Include the relevant issue number if applicable.
+
+#### **Do you intend to add a new feature or change an existing one?**
+
+* Suggest your change in the [Issues](https://github.com/awspring/spring-cloud-aws/issues).
+
+* Start writing code once the issue got approved by project maintainers.
+
+* In addition to actual implemented feature, remember to:
+
+  * update reference docs
+  * consider providing/updating one of the [sample applications](https://github.com/awspring/spring-cloud-aws/tree/main/spring-cloud-aws-samples)
+  * each public class should have a Javadoc
+  * code has to have tests
+  * each package has to have `package-info.java` file definining nullability rules ([example](https://github.com/awspring/spring-cloud-aws/blob/main/spring-cloud-aws-core/src/main/java/io/awspring/cloud/core/package-info.java))
+  * each nullable field, method parameter, method return value, if can be null, has to be annotated with `org.springframework.lang.Nullable`.
+
+#### **Do you have questions about the source code?**
+
+* Ask any question about how to use Spring Cloud AWS in the [Discussions](https://github.com/awspring/spring-cloud-aws/discussions).
+
+Thanks!