Guidelines/philosophy for new operations, including security principles

[quidity]

The universe of operations supported by this spec is likely to grow in the future.

To ensure that new operators are shaped in a way that they can be implemented securely, it would be good to include some general guidelines/philosophy on how ops are defined to reduce potential for ops to result in implementation problems.

For instance, avoid reshaping tensors, simplicity of arguments, no parsers for complex data formats.

[anssiko]

The group discussed this issue and agreed to add "secure by design" guidelines to the spec (possibly to the MLOperand section, cross-linked to dedicated security section as appropriate).

Related, we received general guidelines from the ONNX project and will incorporate also this perspective along with the security-focused guidelines.

[anssiko]

I pushed 4549a67 to PR #251 to add "Guidelines for new operations" section as proposed.

This incorporates input from @quidity as well as from the ONNX project shared at our TPAC 2021 meeting, and some other proposals I thought might be helpful.