Make event section descriptions consistent in Readme.md

This is a non-code PR, but the documentation was bugging me a little.

* Some descriptions were after the code, some were before. Now they are all before the code sample they pertain to.
* `field` has no description, `fileBegin` had `field`'s description, `file` had `fileBegin` and `file`'s descrition. Now they are all in the proper section.