Add an 'Extending Assert' section to the readme

[Gert de Pagter]

There are a few protected static methods, that are allowed to be extended. Adding a section to the readme should help users who would like to configure some behaviour for their own projects.

https://github.com/webmozart/assert/blob/80778ecf9fe61ae2a8e842cc302b92b611475b18/src/Assert.php#L1116-L1203