fix: Change Readme contents to reflect implementation

[ltfschoen]

I had a closer look at the code to see what Node Interface methods we are wrapping in @polkadot/api-observable, and which Node Interface methods we are wrapping in @polkadot/rpc-core, and I think these changes are appropriate.

The link to the package @polkadot/rpc-rx that we are using appears to have been accidently removed previously, so I've added it back again.

Note importantly that @polkadot/api contains @polkadot/api/rx, whereas @polkadot/rpc-rx is a different separate package

[jacogr]

Yes, api-observable needs to -

• start using api/rx
• remove all the single-use methods, only contain wrappers (i.e. all the stuff where combine is used)

rpc-rx, like rpc-core needs to be on this list, but it one of those things we probably don't want use of directly unless people know what they are doing - they only take RPC calls and either make them promises or observables.

EDIT: "accidentally removed" - that is my overzealous editing when I saw the name "api-rx" (all before morning coffee) ;)

[polkadot-js-bot]

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