Add warning about using this package to send arbitrary data to worker threads

[valadaptive]

I confirm that this contribution is made under the terms of the license found in the root directory of this repository's source tree and that I have the authority necessary to make this contribution on behalf of its copyright owner.

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I've discovered that many JavaScript packages use this package to try and send arbitrary data containing functions to worker threads, basically using it as a version of postMessage() that doesn't throw if there's a function in there somewhere. In particular, it seems to be used by developer tooling that wants to "transparently" start running things in parallel, passing a user-supplied configuration object into each worker thread. Here's @rollup/plugin-terser using it, and here it is in Mocha, a popular testing framework.

However, there's a very good reason for postMessage() to throw an error when encountering a function. Functions can, in general, reference data defined outside the function. Serializing that data would be quite difficult, and serialize-javascript just doesn't do it.

Using serialize-javascript to send arbitrary data to worker threads may work in simpler cases, but it fails as soon as the functions reference things like imported modules. I think we should make it clear that it's not magic, and doesn't let you treat worker threads the same way as the main thread. As such, this PR adds a disclaimer pretty high up in the readme.