Update identity-iota description

[cycraig]

Description of change

Brief description for the identity-iota README.md.

• Fix broken intra-doc links.
• Add brief module descriptions.
• Make error module private: Error, Result already exported directly.

Links to any relevant issues

Addresses task of #133

Type of change

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

cargo doc --no-deps --open reports no warnings or errors.

Change checklist

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[olivereanderson]

LGTM!