docs: Event Stream design (#460)

* docs: Event Stream design

Author: ganeshnj

design/rfc-00000-template.md

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+# RFC template
+
+* Inspired from
+  https://github.com/apple/swift-evolution/blob/main/proposal-templates/0000-swift-template.md
+* Keep line length between 80-120 (80 recommended) ignoring URIs/links that
+  can't be broken. Recommended to use
+  https://marketplace.visualstudio.com/items?itemName=stkb.rewrap
+* Include diagrams in your Markdown files with
+  [Mermaid](https://github.blog/2022-02-14-include-diagrams-markdown-files-mermaid/)
+
+* * *
+
+# Feature name
+
+* Proposal: [RFC-00001](design/rfc-00001-event-stream.md)
+* Authors: [Author 1](https://github.com/awsdev),
+  [Author2](https://github.com/awsdev)
+
+* Status: **Awaiting implementation | Implemented  | Review | Rejected
+    **
+
+## Introduction
+
+A short description of what the feature is. Try to keep it to a single-paragraph
+"elevator pitch" so the reader understands what problem this proposal is
+addressing
+
+## Motivation
+
+Describe the problems that this proposal seeks to address. If this is a new
+feature, describe the problems that the feature will solve.
+
+## Proposed solution
+
+Describe your solution to the problem. Provide examples and describe how they
+work. Show how your solution is better than current workarounds: is it cleaner,
+safer, or more efficient?
+
+## Detailed design
+
+Describe the design of the solution in detail. If it's a new API, show the full
+API and its documentation comments detailing what it does. The detail in this
+section should be sufficient for someone who is *not* one of the authors to be
+able to reasonably implement the feature.
+
+## Source compatibility
+
+Will existing SDK applications stop compiling due to this change? Will
+applications still compile but produce different behavior than they used to?
+
+We should only break source compatibility if absolutely necessary. It's better
+to deprecate and provide a migration path.
+
+## Alternatives considered
+
+Describe alternative approaches to addressing the same problem, and why you
+chose this approach instead.
+
+## Acknowledgments
+
+If significant changes or improvements suggested by members of the community
+were incorporated into the proposal as it developed, take a moment here to thank
+them for their contributions. AWS SDK for Swift RFC is a collaborative process,
+and everyone's input should receive recognition!