Branch docs developer guide

[ruilingk]

Update diagrams.

[codecov-io]

Codecov Report

Merging #97 into master will decrease coverage by 0.08%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master      #97      +/-   ##
============================================
- Coverage     72.52%   72.43%   -0.09%     
  Complexity      524      524              
============================================
  Files            83       83              
  Lines          1623     1625       +2     
  Branches        167      167              
============================================
  Hits           1177     1177              
- Misses          400      402       +2     
  Partials         46       46              

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/seedu/tr4cker/ui/MainWindow.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...ava/seedu/tr4cker/ui/planner/PlannerTabWindow.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)

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[cwenling]

Lgtm