Add details for gradle custom plugin.

[prudhvigodithi]

Signed-off-by: pgodithi pgodithi@amazon.com

Custom plugin's readme

Description

This PR will add a doc for gradle custom plugins and explains in details on how to use opensearch.pluginzip plugin

Issues Resolved

https://github.com/opensearch-project/opensearch-plugins/issues/143

Associated issues

https://github.com/opensearch-project/opensearch-build/issues/1916

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[prudhvigodithi]

Hey @dblock can you please review this PR? (To better Read) I will link this doc to the issue's created for each plugin repo as as part of the campaign. Example the issue will be something like this https://github.com/prudhvigodithi/opensearch-build/issues/18

[prudhvigodithi]

Thanks @gaiksaya, added all your suggestions in the latest commits.

[prudhvigodithi]

Hey @dblock, you suggest to move the data of GRADLE_CUSTOM_PLUGINS.md(added via this PR) to BUILDING.md? Also Coming to opensearch-plugin-template-java, this I see for creating any new OpenSearch plugins repo and but not for gradle plugins (under buildSrc/), did I miss anything from you previous comment ?

[prudhvigodithi]

Hey @dblock migrated the details to BUILDING.md. PR for opensearch-plugin-template-java Once this is merged I will add a new PR for opensearch-plugin-template-java to add details about opensearch.pluginzip

[bbarani]

I mostly have nitpicks, the doc is good!

@dblock Can we go ahead and merge this PR and work on the nitpicks later? I would like to see the teams start publishing the plugin zips from 2.0 GA version.

[prudhvigodithi]

Hey @dblock I have pushed some suggested NIT's, please review. Thanks