Add status badge for CI actions

Signed-off-by: mgodwan <mgodwan@amazon.com>

Author: mgodwan

README.md

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+[![Backward Compatibility Checks](https://github.com/opensearch-project/custom-codecs/actions/workflows/ci.yml/badge.svg?branch=main)](https://github.com/opensearch-project/custom-codecs/actions/workflows/ci.yml)
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+[![Publish snapshots to maven](https://github.com/opensearch-project/custom-codecs/actions/workflows/publish-maven-snapshots.yml/badge.svg?branch=main)](https://github.com/opensearch-project/custom-codecs/actions/workflows/publish-maven-snapshots.yml)
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 # Custom Codecs
 Custom Codecs plugin makes it possible for users to provide custom Lucene codecs for loading through Apache Lucene's `NamedSPILoader`.
 These codecs can be used to customize the on-disk representation of the opensearch indexes. For example, zstd compression can be used for