Place for external documentation?

[Chris Johns]

On 24/08/2017 16:49, Sebastian Huber wrote:
> Sorry, a typo.
> 
> On 24/08/17 08:48, Sebastian Huber wrote:
> 
>> For example the PowerPC ABIs are not referenced by the RTEMS CPU Supplement. 
> 
> For example the PowerPC ABIs are now referenced by the RTEMS CPU Supplement.
> 

Ah OK I see where this is coming from now.

Can documents that are not referenced be added?
Any need for a structure under `docs/contrib`? If it grows which it could a lot
of files could become messy.

Chris