README: update PHP extensions
README: update PHP extensions

I have also removed default extensions to be easier to read.
Extensions that are used but enabled by default in PHP could maybe be
listed in an advanced documentation instead.
https://github.com/marienfressinaud/FreshRSS/issues/575

marienfressinaud/FreshRSS,plopoyop/FreshRSS,samuelstein/FreshRSS,samuelstein/FreshRSS,Dolomich/FreshRSS,asmundg/FreshRSS,Dolomich/FreshRSS,Alkarex/FreshRSS,aledeg/FreshRSS,superboum/FreshRSS,aledeg/FreshRSS,FreshRSS/FreshRSS,FreshRSS/FreshRSS,FreshRSS/FreshRSS,Alkarex/FreshRSS,FreshRSS/FreshRSS,romibi/FreshRSS,asmundg/FreshRSS,Dolomich/FreshRSS,asmundg/FreshRSS,marienfressinaud/FreshRSS,romibi/FreshRSS,romibi/FreshRSS,aledeg/FreshRSS,plopoyop/FreshRSS,superboum/FreshRSS,aledeg/FreshRSS,marienfressinaud/FreshRSS,samuelstein/FreshRSS,Alkarex/FreshRSS,marienfressinaud/FreshRSS,romibi/FreshRSS,superboum/FreshRSS,Alkarex/FreshRSS