posts endpoint example code PHP/jQuery

#33

I am submitting this pull request to give context to my questions in #33 about where this should go. Line 1-5 of the PHP example is mostly general information that should go in an introductory section or one of the existing guides.
This gives you the URL for the local installation, but if you're running this locally, it's simpler just to run the code internally. Presumably you'll want external URLs instead.