Add note that CrawlerProcess.crawl() needs a crawler or spider class, not a spider instance

[redapple]

Even if it's in the docs,

crawler_or_spidercls (Crawler instance, Spider subclass or string) – already created crawler, or a spider class or spider’s name inside the project to create it

users still miss sometimes that .crawl() expects a Spider class (or a crawler) but not a Spider instance.

e.g. http://stackoverflow.com/q/39639568

I overlooked the fact that the class itself has to be passed as an argument, and not an instance of that class.
This is quite subtle on the eye but makes all the difference

This ticket is to make it a bit more prominent.