Adding a plugin standards file including broken link checker

Signed-off-by: Vacha Shah <vachshah@amazon.com>

STANDARDS.md

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+# Plugin Standards
+
+This document lists down the standards for OpenSearch plugins which can be used by plugin authors.
+
+- [Plugin Standards](#plugin-standards)
+  - [Broken Links Checker](#broken-links-checker)
+  - [Plugin Release Notes](#plugin-release-notes)
+  - [License Headers](#license-headers)
+
+## Broken Links Checker
+
+Create a Github Actions workflow to check for broken links in text files (.md, .html, .txt, .json) on pull requests and push. See [.github/workflows/links.yml](.github/workflows/links.yml) for an example.
+
+See [lycheeverse/lychee-action](https://github.com/lycheeverse/lychee-action) for more information.
+
+## Plugin Release Notes
+
+Release notes for plugins are located at [RELEASE_NOTES.md](RELEASE_NOTES.md).
+
+## License Headers
+
+License Header details are located at [HEADERS.md](HEADERS.md).