Clarify in doc that CPU fallback will be required

[forderud]

Clarify documentation to state that conformant loaders shall fallback to a CPU-based implementation if sufficient GPU resources are not available.

Not that this does not need to be fully implemented for EuroEcho. It's intended for the final release of loaders.

[SteveKauffman]

minor issue: "EchoEcho" --> "EuroEcho"

[forderud]

Fixed.

[forderud]

I believe that the need for CPU fallback is at least partially addressed by the https://github.com/MedicalUltrasound/Image3dAPI/blob/master/Guidelines.md document, that states:
The loader should still be well behaved when running without a DirectX 11 compliant GPU, but without any performance guarantees