Added a tutorial on using teasers functionality.

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+---
+title: Using teasers in Hakyll
+author: Ivan Veselov
+---
+
+## Introduction
+
+Sometimes it is convenient to have an excerpt of a post displayed on
+the index page along with a "Read more..." link to encourage your
+readers to read the rest of the post.
+
+You can do this in Hakyll by using "teasers" functionality.
+
+## Marking teasers in posts
+
+First, you have to put a tag in your post which separates the teaser
+from the rest of the article. We use `<!--more-->` for this to mimic
+the [WordPress convention](http://codex.wordpress.org/Customizing_the_Read_More):
+
+``` markdown
+---
+title: My teasering post
+---
+In this post I'll try to explain how to use teasers.
+
+<!--more-->
+
+And here I am proceeding with the explanation.
+
+```
+
+This is an HTML comment, so it doesn't have any visual impact on the
+post, it is there solely for semantic purposes.
+
+## Referring to teasers in templates
+
+Now, we want to refer to the teaser in the template. We can do this
+pretty intuitively by using `$teaser$` key:
+
+``` html
+<li class="post_item">
+    $date$ - $title$
+    <p>Teaser: $teaser$</p>
+    <a href="$url$">Read more</a>
+</li>
+```
+
+## Gluing together
+
+The only thing left is to glue those things together. An important
+question here is on what stage of the compilation we want to extract a
+teaser. Usually, we want to use pandoc output, i.e. raw HTML without
+any templates applied (since we do not want some surrounding
+JavaScript or common text from the templates to be included in the
+teaser). We can specify this by using snapshots: we save the snapshot
+during compilation and load it to resolve `$teaser$` key later:
+
+``` haskell
+match "posts/*" $ do
+  route $ setExtension ".html"
+  compile $
+     pandocCompiler
+     -- save immediately after pandoc, but before the templates are applied
+     >>= saveSnapshot "content"
+     >>= loadAndApplyTemplate "templates/post.html" defaultContext
+     ...
+```
+
+You can read more about snapshots in
+[Snapshots tutorial](/tutorials/05-snapshots-feeds.html).
+
+Then we use this snapshot while generating teasers using
+`teaserContext` function:
+
+``` haskell
+    loadAndApplyTemplate
+         "template/postitem.html"
+         (teaserContext "content" <> defaultContext)
+         item
+```
+
+Here, we've just added a new context which knows how to handle
+`$teaser$` key to the default context (note that we passed the same
+snapshot name `"content"` which we used while saving).
+
+## Known issues
+
+Since we use an HTML comment `<!--more-->` to separate the teaser,
+sometimes `pandoc` can "eat" the comment while converting (for example
+this happens with literate Haskell source). In order to overcome this
+problem you may try to use something like this as a separator:
+
+``` html
+<div></div><!--more-->
+```