Add verification steps to README

[handshake-enthusiast]

Useful comprehensive guides:

• https://www.reddit.com/r/Bitcoin/wiki/verifying_bitcoin_core
• https://bitcoincore.org/en/download/
• https://bitcoin.stackexchange.com/questions/50185/how-to-verify-bitcoin-core-release-signing-keys
• https://www.getmonero.org/resources/user-guides/verification-allos-advanced.html
• https://docs.wasabiwallet.io/using-wasabi/InstallPackage.html
• https://help.ubuntu.com/community/HowToSHA256SUM

Re:

• https://github.com/kyokan/bob-wallet/issues/436
• https://github.com/kyokan/bob-wallet/issues/280
• https://github.com/kyokan/bob-wallet/pull/478#issuecomment-1067161090
• https://github.com/kyokan/bob-wallet/issues/214

This new section could be linked/included into release tags.

[handshake-enthusiast]

bob-2.0.0-arm64.dmg wasn't signed BTW.

[handshake-enthusiast]

having a section may persuade a few more to do so.

Indeed. I noticed the SHA256SUMS.asc file and it wasn't straightforward for me how to use it. I'm a semi power user here, not experienced with this stuff, but was able to understand it could be used.

[handshake-enthusiast]

@rithvikvibhu the PR should be ready for another review round. Once you approve I'll squash the commits into one.

[handshake-enthusiast]

Sorry, I used a wrong user and had to delete the repo. Will redo later when not in a hurry.

[handshake-enthusiast]

Unfortunately, after deleting my original fork I can't update this PR, so I had to create a new one. See https://github.com/kyokan/bob-wallet/pull/612. It wasn't my cleanest commit history... A good lesson not to do things in a hurry.