Update docs

[johnfercher]

Description

• Update docs

Related Issue

#386

Checklist

check with "x", if applied to your change

• All methods associated with structs has func (<first letter of struct> *struct) method() {} name style.
• Wrote unit tests for new/changed features.
• Followed the unit test when,should naming pattern.
• Updated docs/doc.go and docs/*
• Updated pkg/pdf/example_test.go
• Updated README.md
• Executed make examples to update all examples inside docs/examples.
• New public methods/structs/interfaces has comments upside them explaining they responsibilities
• Executed make dod with none issues pointed out by golangci-lint

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (319c593) 81.57% compared to head (dd54025) 81.57%.

Files	Patch %	Lines
maroto.go	82.36%	3 Missing ⚠️
metricsdecorator.go	80.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #393   +/-   ##
=======================================
  Coverage   81.57%   81.57%           
=======================================
  Files          61       61           
  Lines        2018     2018           
=======================================
  Hits         1646     1646           
  Misses        345      345           
  Partials       27       27           

Flag	Coverage Δ
unittests	81.57% <81.49%> (ø)

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