Document default handlers

[adamgreig]

After #85 was merged we never updated the documentation to mention the new default handlers. Instead, it still says

It's mandatory to set the HardFault handler somewhere in the dependency graph of an application

which is technically true but a little misleading. We should probably mention the defaults in any event, and possibly update the example in the crate-level documentation to not manually specify the handlers.

[jonas-schievink]

Looks like this was done