Improvements to README.md
Improvements to README.md

1) Some grammatical fixes
2) Add instructions for CentOS / Fedora in the Requirements section

Zaszczyk/cphalcon,unisys12/phalcon-hhvm,unisys12/phalcon-hhvm,unisys12/phalcon-hhvm,unisys12/phalcon-hhvm,Zaszczyk/cphalcon,unisys12/phalcon-hhvm,Zaszczyk/cphalcon,unisys12/phalcon-hhvm,Zaszczyk/cphalcon