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.
It is not strictly related, but the docs above used `sel.xpath('//span/text()')` to extract the text. Discussing `sel.xpath("//a[1]//text()").extract()` as an alternative might be helpful here. 

> When you need to use the text content as argument to a XPath string function

IMHO XPath string function(s) is "intermediate" XPath, and it is not obvious that e.g. `contains` uses `string` under the hood. So for reader it may be unclear when you need to use text content as argument to a XPath string function - should it be used to extract the text, and how is it related to text extraction methods showed earlier in the document? I think that the confusion could make the tip less helpful than it could be.