Documenting how to pass request body and headers

For body:

https://groups.google.com/forum/?fromgroups=#!topic/angular/ps33J7ZQw8c

For headers:

http://pastebin.com/4U6mTgny

It's making a lot of confusion without proper doc!

[btford]

The docs have since been improved, but if there's still an omission, feel free to send a PR. Thanks!