Website: warn against side effects in bare code

[BrainCrumbz]

See example gist.

When running from console, this line runs only once, during context building phase. And not on each example execution.

When running from VS IDE, that line runs everytime test discovery is requested, then everytime test execution is requested. And not on each example execution.

Such a warning should be placed in general in doc page, e.g. in some general conventions, or common rules, or similar. And then also in VS integration section, once PR #163 lands and that section will be needed as well.