[#452] Update README.md

[bixycler]

Clarify the name difference between jaxb2-basics in versions 2.X and jaxb-plugins in versions 3.X and 4.X.

In issue #375, many codes were updated to change from jaxb2-basics to jaxb-plugins, but the docs are left out-of-date. I think at least the README should have a clarification for this change.

Here's the situation for new user like me:

• I read "JAXB Plugins (former JAXB Basics)" and thought "OK, so now it's called JAXB Plugins". But right below it there are subtitles called "Using JAXB Basics" and "JAXB Basics Plugins"... I was confused and thought "OK, It's name 'Basics' there is to distinguish from the main Maven plugin, huh!"
• Then throughout the docs, I only saw jaxb2-basics in the example codes, which lead me to search for "jaxb2-basics" on Maven Central. I only saw versions 2.X but thought "The Basics Plugins are not said to have the same version as the main Maven Plugin, hence the latest version is OK!"
• I ran xjc with -classpath to org.jvnet.jaxb:jaxb2-basics and got error.

[laurentschoelens]

@bixycler thanks for this one
In order to follow up updating documentation in wiki, could you please create an issue here so we can follow up the update and close it by changing the readme section again ?
Thanks

[bixycler]

I've just created the issue #452. For tracking further updates.

[laurentschoelens]

Many thanks. This PR has been merged in master already to help other users that could been confused by the current documentation state