Document lua filters

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+Lua Filters
+===========
+
+Pandoc expects lua files to return a list of filters. The filters in that list
+are called sequentially, each on the result of the previous filter.
+
+Lua Filter Structure
+--------------------
+
+Lua filters are tables with element names as keys and values consisting
+of functions acting on those elements.
+
+Filter Application
+------------------
+
+For each filter, the document is traversed and each element subjected to
+the filter. Elements for which the filter contains an entry (i.e. a
+function of the same name) are passed to lua element filtering function.
+In other words, filter entries will be called for each corresponding
+element in the document, getting the respective element as input.
+
+The element function's output must be an element of the same type as the
+input. This means a filter function acting on an inline element must
+return an inline, and a block element must remain a block element after
+filter application. Pandoc will throw an error if this condition is
+violated.
+
+Elements without matching functions are left untouched.
+
+See [module documentation](pandoc-module.html) for a list of pandoc
+elements.
+
+
+Pandoc Module
+=============
+
+The `pandoc` lua module is loaded into the filter's lua environment and
+provides a set of functions and constants to make creation and
+manipulation of elements easier. The global variable `pandoc` is bound
+to the module and should generally not be overwritten for this reason.
+
+Two major functionalities are provided by the module: element creator
+functions and access to some of pandoc's main functionalities.
+
+Element creation
+----------------
+
+Element creator functions like `Str`, `Para`, and `Doc` are designed to
+allow easy creation of new elements that are simple to use and can be
+read back from the lua environment. Internally, pandoc uses these
+functions to create the lua objects which are passed to element filter
+functions. This means that elements created via this module will behave
+exactly as those elements accessible through the filter function parameter.
+
+Exposed pandoc functionality
+----------------------------
+
+Some filters will require access to certain functions provided by
+pandoc. This is currently limited to the `read` function which allows to
+parse strings into pandoc documents from within the lua filter.
+
+
+Examples
+--------
+
+### Macro substitution.
+
+The following filter converts strings containing `{{helloworld}}` with
+emphasized text.
+
+``` lua
+return {
+  {
+    Str = function (elem)
+      if elem.text == "{{helloworld}}" then
+        return pandoc.Emph {pandoc.Str "Hello, World"}
+      else
+        return elem
+      end
+    end,
+  }
+}
+```
+
+### Default metadata file
+
+Using the metadata from an external file as default values.
+
+``` lua
+-- read metadata file into string
+local metafile = io.open('metadata-file.yaml', 'r')
+local content = metafile:read("*a")
+metafile:close()
+-- get metadata
+local default_meta = pandoc.read(content, "markdown").meta
+
+return {
+  {
+    Meta = function(meta)
+      -- use default metadata field if it hasn't been defined yet.
+      for k, v in pairs(default_meta) do
+        if meta[k] == nil then
+          meta[k] = v
+        end
+      end
+      return meta
+    end,
+  }
+```