Rewrite the section in the philosophy vignette

[chainsawriot]

rio/vignettes/philosophy.Rmd

Line 44 in 6be96d1

8. **rio** imports and exports files based on an internal S3 class infrastructure. This means that other packages can contain extensions to **rio** by registering S3 methods. These methods should take the form `.import.rio_X()` and `.export.rio_X()`, where `X` is the file extension of a file type. An example is provided in the [rio.db package](https://github.com/leeper/rio.db).

Now, one can refer to the "Extending rio" vignette.