docs: clarify usage of plugin is not to be used as standalone

README.md

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 Snyk helps you find, fix and monitor for known vulnerabilities in your dependencies, both on an ad hoc basis and as part of your CI (Build) system.
 
+| :information_source: This repository is only a plugin to be used with the Snyk CLI tool. To use this plugin to test and fix vulnerabilities in your project, install the Snyk CLI tool first. Head over to [snyk.io](https://github.com/snyk/snyk) to get started. |
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 ## Snyk SBT CLI Plugin
 
 This plugin provides dependency metadata for SBT projects that use `sbt` and have a `build.sbt` file.