Add explicit dev setup for easy contributions about mongoose-pii

[tdd]

Note that this is part of growing our quality score on npms / npmjs.com quality index:

quality = 
  carefulness: calculateScore(quality.carefulness, aggregation.carefulness, 0.8),
  tests: calculateScore(quality.tests, aggregation.tests, 0.7),
  health: calculateScore(quality.health, aggregation.health, 1),
  branding: calculateScore(quality.branding, aggregation.branding, 1),

  weightedMean([
    [scores.carefulness, 7], // 0.87, 7 (ESLint config needed)
    [scores.tests, 7],       // 0.85, 7 (CodeCov / Coveralls instead of CodeClimate :-/)
    [scores.health, 4],      // 1.00, 4
    [scores.branding, 2],    // 0.60, 2 (need custom Homepage, e.g. GH Pages)

  // => 0.87 * 7 + 0.85 * 7 + 4 + 0.6 * 2 = 86,2% (npm dit 78% !)
  // Avec ESLint configuré => 90,8%
  // Avec ESLint + custom HP => 94,7%
  // Avec ESLint + custom HP + CodeCov pour les tests => 100%

[tdd]

All these measures have been implemented, closing.