Add section about non-determinism

[axic]

Ref: https://github.com/WebAssembly/design/blob/master/Nondeterminism.md

Should clearly explain how do we handle these cases.

In short: Out of that list only two things affect ewasm:

• NaN on arithmetic operations. This only affects floating point, which we reject at deploy time.
• Resource exhaustion (such as out of memory, etc.). This is handled via the metering process.

Also see #90 and #92.