add CONTRIBUTING.md
add CONTRIBUTING.md

prolic/zend-expressive,ezimuel/zend-expressive,pborreli/zend-expressive,prolic/zend-expressive,nightlinus/zend-expressive,prolic/zend-expressive,mwillbanks/zend-expressive,grizzm0/zend-expressive,pborreli/zend-expressive,kynx/zend-expressive,kynx/zend-expressive,matwright/zend-expressive,weierophinney/zf-docs-toolchain-experiment,jguittard/zend-expressive,zf2timo/zend-expressive,micheh/zend-expressive,pborreli/zend-expressive,matwright/zend-expressive,kynx/zend-expressive,codeliner/zend-expressive,kingjerod/zend-expressive,jguittard/zend-expressive,Stanimirdim92/zend-expressive,moderndeveloperllc/zend-expressive,grizzm0/zend-expressive,matwright/zend-expressive,grizzm0/zend-expressive,mwillbanks/zend-expressive,kingjerod/zend-expressive,acelaya/zend-expressive,jguittard/zend-expressive