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.
The reader may become disappointed because 75.2% of the contents from this link is already in the docs. You read the tips, want more, click the link and get the same tips (unless you scroll to the bottom of the article).

* mention that more explicitly ("tips are based on this blog post")?
* add all remaining tips from the blog post to the docs?