Add segwit upgrade guide

[harding]

This adds a guide to upgrading (or not upgrading) to segwit for miners, node operators, wallet users, and Bitcoin devs.

• A link to this guide is added to the FAQ menu.
• A link is not added to the main page, as it seems like we'll be adding a link there soon enough for the 0.13.1 release.
• All Markdown reference-style links to this document are defined, as verified by searching the rendered document for the string ][ that is indicative of an undefined link.
• It is expected that the release blog for 0.13.1 will link to this document; however, I think it is safe to merge this document prior to the release if everyone is ok with it's content (which is why it's in a separate PR from the upcoming PR for the release blog).

A preview screenshot may be found here: http://imgur.com/a/DZGAg

Several people provided suggestions on this in draft form, for which I am very grateful. Any errors that appear in this version are entirely my fault.

[btcdrak]

double version (again at the bottom)

[kanzure]

ACK 290d0c4. Also possibly consider adding a link to /en/2016/01/26/segwit-benefits/. Looking good.

[btcdrak]

Converted to a blog post, makes it more accessible. Agree with @kanzure to add link to segwit benefits.

[btcdrak]

@harding Thank you very much

[laanwj]

post-merge ACK, thanks!

[kanzure]

Navigation menu link is pointing to /en/segwit-upgrade-guide but permalink is /en/2016/10/27/segwit-upgrade-guide/.

[btcdrak]

@kanzure I dont understand why, the links are updated in the navigation.yml

[harding]

@btcdrak There's a build error:

$ bundle exec jekyll serve --future
Configuration file: /home/user/bitcoincore.org/_config.yml
            Source: /home/user/bitcoincore.org
       Destination: /home/user/bitcoincore.org/_site
 Incremental build: disabled. Enable with --incremental
      Generating... 
jekyll 3.0.2 | Error:  (/home/user/bitcoincore.org/_data/navigation.yml): did not find expected key while parsing a block mapping at line 128 column 6

[btcdrak]

@harding thanks, indentation was wrong. Fixed.