Adding some xpath tips to selectors docs

Hey, folks!

I've chosen some of XPath tips from the blog post at http://blog.scrapinghub.com/2014/07/17/xpath-tips-from-the-web-scraping-trenches/ that I thought were relevant to be in the official docs.

I kept the big ones and left a link for the blog post and the XPath tutorial.

Please let me know what you think.