Add README case: address from private key

[abdulowork]

Lets add a case for accessing address from a private key via EthPrivateKey and PrivateKeyAddress as a getter

[0crat]

The job #129 assigned to @biboran/z, here is why; the budget is 30 minutes, see §4; please, read §8 and §9; if the task is not clear, read this and this; we should be aware that @biboran/z is on vacation; this ticket may be delayed; there will be no monetary reward for this job

[0crat]

Job gh:blockstoreapp/web3swift#129 already assigned to @biboran, can't assign to @biboran

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Bug was reported, see §29: +15 point(s) just awarded to @biboran/z

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It is strongly discouraged to assign jobs to their creators, see §19: -15 point(s) just awarded to @biboran/z

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Manual assignment of issues is discouraged, see §19: -5 point(s) just awarded to @biboran/z