Update Getting Started + README

[cgarciae]

• Getting Started: Basic API
• Getting Started: Low-level API
• README

[codecov-io]

Codecov Report

Merging #152 (d1f9048) into master (a8b917b) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #152      +/-   ##
==========================================
- Coverage   84.84%   84.83%   -0.01%     
==========================================
  Files         131      131              
  Lines        6796     6794       -2     
==========================================
- Hits         5766     5764       -2     
  Misses       1030     1030              

Impacted Files	Coverage Δ
elegy/model/model_base.py	83.47% <ø> (-0.95%)	⬇️
elegy/model/model_core.py	85.00% <ø> (-0.72%)	⬇️
elegy/generalized_module/elegy_module_test.py	100.00% <100.00%> (ø)
elegy/generalized_module/haiku_module_test.py	100.00% <100.00%> (ø)
elegy/generalized_module/linen_module_test.py	100.00% <100.00%> (ø)
elegy/model/model.py	96.47% <100.00%> (+0.01%)	⬆️
elegy/module.py	95.32% <100.00%> (ø)
elegy/module_test.py	99.25% <100.00%> (+<0.01%)	⬆️
elegy/nets/resnet_test.py	100.00% <100.00%> (ø)
elegy/generalized_module/generalized_module.py	66.15% <0.00%> (-1.54%)	⬇️
... and 2 more

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