README references old instructions

[Jonathan-Zollinger]

We need to update the README to

• reflect GRAALVM-CE21 (instead of 17)
• show where to find docs on usage for this tool (if available by the time this is carried out)
• show where to download the latest version
• reference the proper authentication variable name
• remove the warning blurb about gradle and the graalvm being resource heavy
• there's no submodule to call when building (not cli:nativeCompile)

[HMS-Victory]

• No docs have been created yet for using the cli tool
• don't know what the new environment variables name is supposed to be

[Jonathan-Zollinger]

@HMS-Victory will you submit your work as a PR to the 0.0.2 branch?

[HMS-Victory]

This was fixed in #28