chore: info about AWS credentials in gitpod to contribution guide (aw…

…s#8474)

Added some infos regarding how to put credentials into gitpod as persisting environment variables. Setting up the CDK dev environment is pretty tough compared to other OSS projects no matter which way you go. Every  infos making it easier should be made available.

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

CONTRIBUTING.md

@@ -184,6 +184,17 @@ Integration tests perform a few functions in the CDK code base -
 3. (Optionally) Acts as a way to validate that constructs set up the CloudFormation resources as expected. A successful
    CloudFormation deployment does not mean that the resources are set up correctly.
 
+For Gitpod users only! The best way to supply CDK with your AWS credentials is to add them as
+[persisting environment variables](https://www.gitpod.io/docs/environment-variables).
+Adding them works as follows via terminal:
+
+```shell
+eval $(gp env -e AWS_ACCESS_KEY_ID=XXXXXXXXX)
+eval $(gp env -e AWS_SECRET_ACCESS_KEY=YYYYYYY)
+eval $(gp env -e AWS_DEFAULT_REGION=ZZZZZZZZ)
+eval $(gp env -e)
+```
+
 If you are working on a new feature that is using previously unused CloudFormation resource types, or involves
 configuring resource types across services, you need to write integration tests that use these resource types or
 features.