[techno review] Update php docs (readme) - about extensions

[zbal]

PHP often require custom extensions to be installed - need to add a section in the documentation that gives some example.

Basically we have 2 options:

• extensions: [ext1, ext2] -- for extensions installed by a regular apt-get
• pecl_extensions: [ext1, ext2] -- for extensions installed by pecl install

I do not believe there is any authoritative listing of the extensions other that google and php.net.

Anyway it needs to be put at the top of the doc so users don't get confused.