add verifying a merkle tree

[ZhouHansen]

🔦 documentation change

Add how to verify a merkle tree to ch01-02-merkle-tree.md

Checklist

• [x] documentation is changed or added

Context

No

Semver Changes

a patch

[yoshuawuyts]

@ZhouHansen this makes a lot of sense; thanks for writing!

[ZhouHansen]

I only know a little bit of verifying a merkle tree. I want to know how bitfield works with merkle tree, but found no doc.