Add details about "linkloan" command enhancement to Developer Guide

[kjw142857]

Fixes #82.

This is a documentation change which updates the description of the linkloan command in the DG, which enables users to link a loan to a target person.

Some highlights:

• Description of the LinkLoanCommand class and its logic flow
• Design considerations for execution of a linkloan command and how the parameters are stored

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.69%. Comparing base (b95a0f2) to head (1067ca4).

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #83   +/-   ##
=========================================
  Coverage     60.69%   60.69%           
  Complexity      462      462           
=========================================
  Files            87       87           
  Lines          1865     1865           
  Branches        183      183           
=========================================
  Hits           1132     1132           
  Misses          681      681           
  Partials         52       52           

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[xiaorui-ui]

lgtm!