Usage as project plugin

[anilkumarmyla]

@rtimush Adding a README section for usage as project plugin. Let me know if this is helpful

[rtimush]

In general, I try to discourage people from using sbt-updates as a project plugin and would rather promote a global installation. It works better with listing the plugin updates, doesn't force you to add the plugin to every project you work on and helps to avoid conflicts between the project and the global plugins.
Nevertheless, there might be cases when you want to add sbt-updates to the project, and then the hints you have about adding it to both project/ and project/project/ come handy. Given that, if you add a warning to the section you added that the global installation method is the preferred one, I will be happy to merge the PR.
Thanks for your contribution!

[anilkumarmyla]

Thanks for the feedback, updated the readme to prefer global installation

[rtimush]

Great, thanks!