Clarify safety on untrusted code

[afq984]

starlark/README.md

Lines 36 to 37 in ce1fdb0

	* **Hermetic execution**. Execution cannot access the file system, network,
	system clock. It is safe to execute untrusted code.

The readme says:

It is safe to execute untrusted code.

google/starlark-go#241 (comment) says:

we've never claimed that it is secure for running untrusted code. Scripts can easily cause denial of service by exhausting all memory, or by hash flooding.

It seems like executing arbitrary starlark code could crash a system, but other than that, there should be no way to escape the execution environment. Is this expectation correct?

The safety expectations also sound different from what https://github.com/google/cel-go offers, so it would be great if this could be elaborated in README.

[stepancheg]

(Deleted wrong comment)