Blog: Add Blog To Bitcoin.org

* Adds new page /en/blog

* Adds new feed for blog posts to /en/rss/blog.xml

* Adds "Blog" link to site footer

* Adds three back-dated blog posts to give the blog some initial
  content.

README.md

@@ -612,3 +612,34 @@ aliases:
   --author=Satoshi Nakamoto: Satoshi Nakamoto
   gavinandresen: Gavin Andresen
 ```
+
+### Blog Posts
+
+Posts for the [Bitcoin.org Site Blog][] should be added to the `_posts`
+directory with the naming convention:
+`YEAR-MONTH-DAY-ARBITRARY_FILE_NAME` (with year, month, and day as
+two-digit numbers).  The YAML front matter should be similar to this:
+
+    ---
+    type: posts
+    layout: post
+    lang: en
+    category: blog
+
+    title: "Quarterly Report March 2015"
+    permalink: /en/posts/quarterly-report-march-2015.html
+    date: 2015-03-05
+    author: >
+      David A. Harding (<a href="mailto:dave@dtrt.org">email</a>, <a
+      href="https://github.com/harding">GitHub</a>,
+      <a href="http://www.reddit.com/user/harda/">Reddit</a>)
+    ---
+
+The type, layout, and category should always be as specified above. The
+other parameters should be set to values specific to that post, but the
+permalink must end in '.html'.
+
+Below the YAML front matter, enter the content of the post in Markdown
+format.  Images should be placed in `img/blog/free` if they are
+MIT-licensed or `img/blog/nonfree` if they have a more restrictive
+copyright license.