[wp-docs] New page - Moving to PHP 5

[mrmist]

As discussed in the #wordpress-dev meet, I've created a codex page

http://codex.wordpress.org/Switching_to_PHP5

Which discusses upgrading from PHP4. I've not yet linked it from the 
main TOC awaiting further editing/refinement/addition/removal/approvals.

Please adjust it as you see fit. I'll add a link from the main TOC once 
there is consensus of opinion, if noone beats me to it.
-- 
mrmist