docs: describe hash -d trick in readme

[psprint]

Description

In #390

Motivation and Context

Related Issue(s)

390 #381

Usage examples

cd ~tigsuite
cd ~zinit-annex-bin-gem-node

How Has This Been Tested?

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[vladdoster]

@psprint,

Could this be used in Zinit functions?

[psprint]

@vladdoster what do you mean?

[psprint]

Zsnap already uses this for all its plugins… Could the PR be merged?