improve README structure and documentation content

[submarcos]

improve #39

[codecov]

Codecov Report

Merging #54 into master will increase coverage by 0.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
+ Coverage   95.75%   96.08%   +0.32%     
==========================================
  Files          38       41       +3     
  Lines        1720     2017     +297     
==========================================
+ Hits         1647     1938     +291     
- Misses         73       79       +6     

Impacted Files	Coverage Δ
geostore/views/__init__.py	96.17% <0.00%> (-0.93%)	⬇️
geostore/settings.py	100.00% <0.00%> (ø)
geostore/routing/decorators.py	100.00% <0.00%> (ø)
...ore/routing/management/commands/update_topology.py	100.00% <0.00%> (ø)
geostore/routing/db/mixins.py	100.00% <0.00%> (ø)
geostore/routing/serializers/mixins.py	87.50% <0.00%> (ø)
geostore/models.py	98.59% <0.00%> (+0.27%)	⬆️
geostore/routing/helpers.py	96.46% <0.00%> (+0.51%)	⬆️
geostore/serializers/__init__.py	97.95% <0.00%> (+0.55%)	⬆️
geostore/tiles/mixins.py	91.97% <0.00%> (+0.84%)	⬆️
... and 2 more

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