Document what's different over built-in 16.1 functionality

[clairernovotny]

VS 2019 16.1 added support for completion of types not in scope. It could be helpful to document what this extension does beyond what VS has built-in so we understand what it does.

Thanks!

[Dreamescaper]

Certainly should be done properly. Will try to do that in nearest future.
Brief description:

• 16.1 added types completion (experimental). It is faster (not dramatically, but still) than provided by this extension, but has some minor bugs. If you're using native completion, makes sense to disable types completion in IntelliSense Extender options.

• There are no extension methods support so far (probably will be added in 16.2 or 16.3)

• No new completions, locals completions (that's something that IntelliCode partially does, but not everything)