Update DG with details of export implementation

[yanchenglee98]

Add description of the implementation of export feature in DG
Closes #102

[codecov-io]

Codecov Report

Merging #103 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #103      +/-   ##
============================================
+ Coverage     72.29%   72.32%   +0.02%     
- Complexity      454      463       +9     
============================================
  Files            79       82       +3     
  Lines          1462     1485      +23     
  Branches        151      152       +1     
============================================
+ Hits           1057     1074      +17     
- Misses          359      364       +5     
- Partials         46       47       +1     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/seedu/address/model/Model.java	100.00% <0.00%> (ø)	1.00% <0.00%> (ø%)
...rc/main/java/seedu/address/model/ModelManager.java	100.00% <0.00%> (ø)	24.00% <0.00%> (+1.00%)
.../java/seedu/address/model/event/AttendeesList.java	66.66% <0.00%> (ø)	1.00% <0.00%> (?%)
...ava/seedu/address/model/event/UniqueEventList.java	85.71% <0.00%> (ø)	2.00% <0.00%> (?%)
src/main/java/seedu/address/model/event/Event.java	55.55% <0.00%> (ø)	3.00% <0.00%> (?%)
src/main/java/seedu/address/model/AddressBook.java	92.30% <0.00%> (+0.64%)	13.00% <0.00%> (+1.00%)
...rc/main/java/seedu/address/logic/LogicManager.java	76.19% <0.00%> (+1.19%)	4.00% <0.00%> (+1.00%)

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[schoolex]

There are still some design format that we still need to discuss

[schoolex]

Almost there, 1 typo to fix