How to use casper-client for smart-contract verification

[moubctez]

What does this PR fix/introduce?

Reproducibility: how to use rustc wrapper

• [*] Docs are successfully building - yarn install && yarn run build.
• [*] For new internal links I used relative file paths (with .md extension) - e.g. ../../faq/faq-general.md - instead of introducing absolute file path, or relative/absolute URL.
• [*] All external links have been verified with yarn run check:externals.
• [*] My changes follow the Casper docs style guidelines.
• [*] All technical procedures have been tested (if you want help with this, mention it in Reviewers).
• [*] If structural changes are introduced (not just content changes), cross-broswer testing has been completed.

Reviewers

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[przemyslaw]

Adds information about using the verify command of casper-client.

[ipopescu]

I've submitted some editing requests. I'll leave the approval to @ACStoneCL.

[ACStone-MTS]

LGTM.

[ipopescu]

Let's merge this so it gets ported.