Improve README to include a general build section

[snicoll]

Is your feature request related to a problem? Please describe.
Cloning this repository and then invoking ./gradlew build on my machine leads to:

FAILURE: Build failed with an exception.

* What went wrong:
A problem occurred configuring project ':native-gradle-plugin'.
> Failed to calculate the value of task ':native-gradle-plugin:compileJava' property 'javaCompiler'.
   > No matching toolchains found for requested specification: {languageVersion=11, vendor=any, implementation=vendor-specific} for MAC_OS on aarch64.
      > No locally installed toolchains match and toolchain download repositories have not been configured.

* Try:
> Learn more about toolchain auto-detection at https://docs.gradle.org/8.3/userguide/toolchains.html#sec:auto_detection.
> Learn more about toolchain repositories at https://docs.gradle.org/8.3/userguide/toolchains.html#sub:download_repositories.
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Run with --scan to get full insights.
> Get more help at https://help.gradle.org.

BUILD FAILED in 1s
49 actionable tasks: 15 executed, 34 from cache

I don't have any particular setup regarding the toolchain that I know of. Given that it's used, I was expecting it to either download the necessary release for me or that some instructions in the README would tell me what to do.

Describe the solution you'd like
With the help of @sdeleuze I was able to actually build quite easily by issuing a single command prior to the build:

$ sdk env install

Not everybody uses SDKMan! I suppose so it would be nice that the README would at least explain what is required and the easiest way to do so.

[vjovanov]

Thanks for reporting. In top-level README.md we need to point developers to the individual plugins for building instructions and mention which version of the JDK they need.