[PROPOSAL] Better readme instructions

[jholla-atx]

Readme for installing plugins is not clear on how to do it.

[dblock]

Does the public documentation help? https://opensearch.org/docs/latest/install-and-configure/plugins/. If so we should link it from README. Generally that's the guide for users.

For developers we can put things in README. Someone recently asked how to write a plugin on Slack and I pointed to https://code.dblock.org/2023/09/29/writing-opensearch-plugins-and-extensions.html. It shows how to do build a plugin with a RESTful API (and an extension in Java and Python). We should improve the README here taking some of that in.

Care to help add/edit what we have here @jholla-atx?