Add warning to custom memory types

[hannahfluch]

When using custom memory types in an uefi application there is a bug that causes the system to freeze on some machines. I had the same issue and struggled for quite sometime. #1375

Thus, I think it would be helpful to add a warning to the documentation regarding this issue:

For the memory management functions in EFI, an OS is meant to be able to use "memory type" values above 0x80000000 >for its own purposes. In the OVMF EFI firmware release "r11337" (for Qemu, etc) there is a bug where the firmware >assumes the memory type is within the range of values defined for EFI's own use, and uses the memory type as an array >index. The end result is an "array index out of bounds" bug; where the higher memory type values (e.g. perfectly legal >values above 0x80000000) cause the 64-bit version of the firmware to crash (page fault), and cause incorrect "attribute" >values to be reported by the 32-bit version of the firmware. This same bug is also present in whatever version of the EFI >firmware VirtualBox uses (which looks like an older version of OVMF); and I suspect (but don't know) that the bug may be >present in a wide variety of firmware that was derived from the tianocore project (not just OVMF).
Source

It is a small change, but can still save new devs a lot of time & frustration.

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[nicholasbishop]

We're in the process of relicensing the project and need signoff on the licensing change from all new contributions. Please take a look at #1470 and add your approval if you're OK with it. Thanks!