docs: try and orient docs more towards beginners

[adamdecaf]

Diff without whitespace: https://github.com/moov-io/ach/pull/379/files?utf8=%E2%9C%93&diff=unified&w=1

Issue: #380

[codecov-io]

Codecov Report

Merging #379 into master will decrease coverage by 0.46%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #379      +/-   ##
==========================================
- Coverage    85.2%   84.73%   -0.47%     
==========================================
  Files         107      107              
  Lines        6684     8673    +1989     
==========================================
+ Hits         5695     7349    +1654     
- Misses        712     1047     +335     
  Partials      277      277

Impacted Files	Coverage Δ
test/ach-axt-read/main.go	57.14% <0%> (-8.08%)	⬇️
test/ach-ccd-read/main.go	57.14% <0%> (-8.08%)	⬇️
test/ach-adv-read/main.go	53.84% <0%> (-8.06%)	⬇️
test/ach-iat-read/main.go	60% <0%> (-8%)	⬇️
test/ach-trc-read/main.go	47.82% <0%> (-7.73%)	⬇️
test/ach-trx-read/main.go	47.82% <0%> (-7.73%)	⬇️
test/ach-xck-read/main.go	47.82% <0%> (-7.73%)	⬇️
test/ach-web-read/main.go	65.71% <0%> (-7.62%)	⬇️
test/ach-shr-read/main.go	45.45% <0%> (-7.49%)	⬇️
test/ach-pop-read/main.go	45.45% <0%> (-7.49%)	⬇️
... and 93 more

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