DL10 documentation

[Phil Budne]

I wrote:
> Now, simulating the DL10 so you can run TVs would REALLY be bringing
> back a lost artifact!!

https://github.com/PDP-10/its/issues/279
says:

 larsbrinkhoff commented on Mar 31, 2017

 I think the number of PDP-11s connected were limited by the address space of the Rubin IO-11 interface. It occupied one of AI's four mobies. One moby can address 16 64k spaces, but somehow I recall seeing the number 8 as the maximum.