More documentation

[joeribekker]

• Partially fixes As productowner, I want to comply to the VNG checklist for standardization #98 -- Added VNG compliancy checklist
• Fixes As productowner, I want to mention related projects in the documentation #65 -- Added background info
• Fixes As productowner, I want an objecttype schema (1) definition to start with #5 and As productowner, I want another objecttype schema (2) definition to start with #6 -- Added examples

[codecov-io]

Codecov Report

Merging #104 (611233f) into master (72b9391) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #104   +/-   ##
=======================================
  Coverage   90.77%   90.77%           
=======================================
  Files          64       64           
  Lines        1496     1496           
=======================================
  Hits         1358     1358           
  Misses        138      138           

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