Update UML sequence diagram for linkloan command

[kjw142857]

Fixes #67.

Adds a UML sequence diagram for the execution of the linkloan command, which includes the Logic and Model components.

Format is similar to the sequence diagram for delete, as shown here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.21%. Comparing base (f2262c9) to head (5a82071).

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #68   +/-   ##
=========================================
  Coverage     62.21%   62.21%           
  Complexity      442      442           
=========================================
  Files            84       84           
  Lines          1736     1736           
  Branches        166      166           
=========================================
  Hits           1080     1080           
  Misses          611      611           
  Partials         45       45           

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[xiaorui-ui]

Do analyze what happens in parseIndex and parseLoan methods(Index and LinkLoanDescriptor)

Optional: What happens between getFilteredPersonList and CommandResult?

[kjw142857]

Thanks for reviewing! The diagram mainly traces out the sequence flow in the Logic and Model components. For other components (e.g. Person/Loan, Commons), I'm planning to add another sequence diagram for this in the future!

[xiaorui-ui]

lgtm!