mention how to import spec files in CONTRIBUTING.md

[trusktr]

Need to add a blurb about the test entry points and all spec files being imported.

[StEvUgnIn]

You can assign me this one. But I need more information than to just 'import'. Do you mean how to write proper tests in Typescript?

[trusktr]

@StEvUgnIn Hello! What I meant was, that currently the spec files are imported, starting with src/as/index.spec.as. If we forget to import a spec file, it will be skipped from tests.

Did it this way because it is way faster (as-pect will make a single Wasm module). The other way is to avoid import altogether and put a file matching pattern in as-pect.config.js so that it picks all the files up, then compiles each one as its own entry point (about 300% slower).