docs(ecs): add contributing guide (#14672)

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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*

packages/@aws-cdk-containers/ecs-service-extensions/CONTRIBUTING.md

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+See: [Contributing Guide](https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/aws-ecs/README.md)

packages/@aws-cdk/aws-ecs-patterns/CONTRIBUTING.md

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+See: [Contributing Guide](https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/aws-ecs/README.md)

packages/@aws-cdk/aws-ecs/CONTRIBUTING.md

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+# Contributing to the AWS ECS and AWS ECS Patterns modules
+
+Hiya! Thanks for your interest in contributing to the ECS modules! The [ECS
+Developer Experience](https://github.com/orgs/aws/teams/aws-ecs-devx) team
+currently owns the following construct libraries:
+
+- [@aws-cdk/aws-ecs](https://github.com/aws/aws-cdk/tree/master/packages/%40aws-cdk/aws-ecs):
+  the main construct library for AWS ECS
+- [@aws-cdk/aws-ecs-patterns](https://github.com/aws/aws-cdk/tree/master/packages/%40aws-cdk/aws-ecs-patterns):
+  a set of simplified, higher-level constructs based on common container-based
+application architectures. Great for first-time container developers!
+- [@aws-cdk-containers/ecs-service-extensions](https://github.com/aws/aws-cdk/tree/master/packages/%40aws-cdk-containers/ecs-service-extensions):
+  a set of ECS constructs that promote best practices for container
+infrastructure by using composable add-ons, such as load balancers and sidecar
+containers used for tracing and metric logging. More suitable for advanced
+container configuration.
+
+## Find something to work on
+
+Issues related to ECS, ECS Patterns, and ECS Service extensions are tracked on
+our public [project board](https://github.com/aws/aws-cdk/projects/2).
+
+If you want to contribute a specific feature or fix you have in mind, check our
+[in-flight work](https://github.com/aws/aws-cdk/projects/2#column-8268897) or
+[open pull requests](https://github.com/aws/aws-cdk/projects/2#column-11918985)
+to see if someone else is already working on it. If an issue has someone
+assigned to it in our "In Progress" column, that means they are actively
+working on it. Otherwise, any unassigned issue is up for grabs!
+
+If an issue doesn't exist for your feature/fix, please create one using the
+appropriate [issue
+template](https://github.com/aws/aws-cdk/tree/master/.github/ISSUE_TEMPLATE).
+
+If you're simply looking for any issue to work on, explore our [Backlog of
+issues](https://github.com/aws/aws-cdk/projects/2#column-8114389) on the public
+project board and find something that piques your interest.  If you are looking
+for your first contribution, the ['good first issue'
+label](https://github.com/aws/aws-cdk/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22)
+will be of help.
+
+### Let us know what you're working on!
+Once you've chosen to work on an existing issue, please **add a comment to
+indicate that you are starting work on it!** This will help us assign the
+issue to you and keep track of issues being actively worked on, to avoid
+duplicate effort.
+
+### Include a design if you can!
+For larger features, your contribution is far more likely to be accepted if you:
+1. let us know you are working on it!
+2. include a design document.
+
+Examples of past designs for the ECS module can be found in under the
+[design](https://github.com/aws/aws-cdk/tree/master/design/aws-ecs) directory.
+
+## Breaking Changes
+See guidance on breaking changes in the [Contributing Guide](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md#breaking-changes).