Add information about specs and hardware compatibility of older XT and 286 machines.

[Fenix770]

Is your feature request related to a problem? Please describe.

No. It's not a problem. I think would be interesting to add information (maybe a document file) about old machines, especially, machines older than 1990. They could be useful for the users and also would help to distinguish bug from normal behaviors of the emulated hardware. (I know it's a lot of information because are serveral different machines emulated)

For example (Other aspects may exist, they are some ideas I have right now)

1. If they have onboard hardware such as RTC clocks or EMS chipsets.
2. The compatibility of the XT machines with high density floppy disk controllers.
3. Which hard disk controller is supported by specific XT machine.
4. Basic information about the capabilities of the EMS controllers and EMS boards emulated
5. Hard disk geometry (C/H/S) on 286/386 machines if they do not provide information on setup BIOS.

Explanation:

2. For example, I prevoiusly though the high density did not work. But I discovered there was not a bug. The problem was some XT machines (such as Amstrad XT) always checks and uses it's own internal floppy drive and ignores any external FDC, the errors displayed in these cases can be confused with bugs on the emulator.
   Others machines such as Tandy 1000 SL/2 and Olivetti M240 works just fine with these controllers.

3. Not all controllers works with all XT machines

4. There are some machines with onboard EMS support and two EMS boards. For example Everex RAM Deluxe and MMF 5150T are LIM EMS 3.2 compatible at hardware level only. Even with a LIM 4.0 drivers they do not support Large Page Frame (used on Windows 2.0/3.0 in real mode and Deskview) in these cases the standard 64kb page frame is used. Also, these cards cannot map UMB memory with applications softwares, unless the 64kb page frame is disabled, because can map only one 64kb region.
   On the other hand, some high end EMS boards (not emulated) can map several segments of upper memory with RAM. In these cases software such as QRAM/ Qualitas Move'em and TLB can be useful, because can map upper memory without disabling EMS memory. Upper memory on the 286 machines was important to save conventional memory in Windows 2.0 for Windows applications, also the Large Page Frame cannot be used to store device drivers, and regular conventional memory was needed.