Update Developer Guide for Vinh

[IceWizard4902]

Same as title.
Add documentation for clear command enhancement and JsonAdaptedEvent for saving events to a Json format

[codecov-commenter]

Codecov Report

Merging #180 (6f144a8) into master (9607c21) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #180   +/-   ##
=========================================
  Coverage     70.51%   70.51%           
  Complexity      613      613           
=========================================
  Files            95       95           
  Lines          1899     1899           
  Branches        245      245           
=========================================
  Hits           1339     1339           
  Misses          461      461           
  Partials         99       99           

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