Add Contributing section to README.md
Add Contributing section to README.md

phpseclib/phpseclib,splitice/phpseclib,paragonie-scott/phpseclib,metaclassing/phpseclib,kenichiii/phpseclib,danog/phpseclib,thaigialai1987/phpseclib,kenichiii/phpseclib,creatuity/phpseclib,melya/phpseclib,splitice/phpseclib,paragonie-scott/phpseclib,thaigialai1987/phpseclib,melya/phpseclib,metaclassing/phpseclib,danog/phpseclib,creatuity/phpseclib,terrafrost/phpseclib