The README file was describing an outdated way of defining serialization
formats. It was updated accordingly to match the current API.
- New format declaration using `setReader` and `setWriter`
and mandatory `PreferredOutput` type declaration,
as introduced in f9a1121b8733eb75e624ab59f8d79e707f15f76f.
- Removal of `fileExt` declaration, which was never implemented.
- Removal of format-specific `dontSerialize` pragma
as well as the `serialize` pragma, which were never implemented.
- Rename `serializedFields` macro to `setSerializedFields`,
to keep up with 60613bac5db2bd7914d484874201f31b3f318165.
- Remove rename functionality in `setSerializedFields`,
which was never implemented.
- A couple updated argument lists and typo corrections.
Other changes:
* The format declarations have been broken down in multiple
parts. This makes it possible to structure the implementation
libraries in a way such that the format name is accessible even
when importing only reader or writer modules.
* Flavors lead to more complicated Reader and Writer types.
Instead of requiring the user to write down long type names
such as `JsonReader[DefaultFlavor]`, it's now possible to refer
to the reader and writer type of each format using expressions
such as `Json.Reader` and `Json.Writer`.
* Fixed a typo (PreferredOutput)
