Add some more details to the plugins docs

[josephjunker]

This adds some additional information on plugins. If the note on the diagnostics availability in early build steps is too much detail I can remove it.

[TwitchBronBron]

This isn't quite true. You can push diagnostics to the program itself with program.addDiagnostics(), but you would need to add a "fake" file. I'm doing this in util.ts as an example.

let diagnostic = {
    message: "Some diagnostic',
    code: 'some-code',
    file: {
        srcPath: configFilePath
    },
    range: util.createRange(1,2,3,4)
} as BsDiagnostic;

But yeah, this pattern isn't overly intuitive, but I don't think this paragraph is quite correct, so I'd say let's remove it, or add my above example as an alternative.

[josephjunker]

That makes sense. I've removed this paragraph, because explaining it might be more confusing then not mentioning it at all.

[TwitchBronBron]

Thanks so much for this!