Define the public-use function yas/create-php-snippet

This implements the function meant for public use, i.e. the function
users should bind to a key in order to use php-auto-yasnippets.
Add extra comments and format the code into sections

This patch contains no changes to any code or logic.  It only divides
the code into sections (i.e. pages) and adds a little commentary for
each section.
Add installation instructions to the README
Update the version number to 0.3.0
Add a note regarding the current development status to the README