custom-writers.md: use filter to include source of example.

doc/custom-writers.md

@@ -34,6 +34,8 @@ can do
 pandoc --print-default-data-file sample.lua > sample.lua
 ```
 
+# A custom HTML writer
+
 `sample.lua` is a full-features HTML writer, with explanatory
 comments. To use it, just use the path to the custom writer as
 the writer name:
@@ -46,3 +48,5 @@ pandoc -t sample.lua myfile.md
 writer, so you can design your own custom writer by modifying
 the functions in `sample.lua` according to your needs.
 
+``` {.lua include="sample.lua"}
+```