Document CrawlSpider.parse_start_urls method

The parse_start_urls method is very useful when you want to scrape paged lists, since you'd need to parse the first page as well as the next ones (which usually get a callback from the Rules). It should make it easier for people asking this very question: http://stackoverflow.com/questions/15836062/scrapy-crawlspider-doesnt-crawl-the-first-landing-page