Update documentation: AboutUs, README, PPP, DG, UG

[coveralls]

Pull Request Test Coverage Report for Build 482

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 26 unchanged lines in 5 files lost coverage.
• Overall coverage remained the same at 81.648%

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Files with Coverage Reduction	New Missed Lines	%
file:/Users/travis/build/AY1920S1-CS2103T-T11-1/main/src/main/java/seedu/moolah/logic/commands/budget/EditExpenseFromBudgetCommand.java	1	97.67%
file:/Users/travis/build/AY1920S1-CS2103T-T11-1/main/src/main/java/seedu/moolah/model/budget/BudgetWindow.java	1	98.57%
file:/Users/travis/build/AY1920S1-CS2103T-T11-1/main/src/main/java/seedu/moolah/model/MooLah.java	4	89.9%
file:/Users/travis/build/AY1920S1-CS2103T-T11-1/main/src/main/java/seedu/moolah/logic/LogicManager.java	8	49.02%
file:/Users/travis/build/AY1920S1-CS2103T-T11-1/main/src/main/java/seedu/moolah/model/ModelManager.java	12	93.1%
<!--	Total:	26		-->

Totals
Change from base Build 480:	0.0%
Covered Lines:	4062
Relevant Lines:	4975

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💛 - Coveralls