Update README for inclusion in release archives

Author: tarleb

CONTRIBUTING.md

@@ -3,7 +3,7 @@ Contributing to lua-filters
 
 Happy to have you here! Below you can find guidance on the
 best way to contribute to this collection of Lua filters.
-All contributions are welcome!
+Contributions are welcome!
 
 Bugs reports and feature requests
 ---------------------------------

README.md

@@ -5,21 +5,11 @@
 [GitHub Actions badge]: https://img.shields.io/github/workflow/status/pandoc/lua-filters/CI?logo=github
 [GitHub Actions]: https://github.com/pandoc/lua-filters/actions
 
-This repository collects Lua filters for pandoc.
+A collection of Lua filters for pandoc.
 
-To learn about Lua filters, see the
-[documentation](http://pandoc.org/lua-filters.html).
+To learn about Lua filters, see the [documentation].
 
-Structure
----------
-
-Each filter goes in its own subdirectory.  Each subdirectory contains:
-
-- the filter itself (e.g. `wordcount.lua`)
-- a `README.md` describing the use of the filter
-- a `Makefile` with a `test` target to test the filter
-- some data files used for the tests, which may also serve
-  as examples
+[documentation]: http://pandoc.org/lua-filters.html
 
 Requirements
 ------------
@@ -30,8 +20,10 @@ is no guarantee that filters will work with older versions, but
 many do.
 
 Some filters depend on external programs, which must be installed
-separately.  Refer to the filters' README for detailed
-requirements.
+separately. Refer to the filters' README for detailed
+requirements; the filter READMEs are not included in the release
+archives, but available online at
+<https://github.com/pandoc/lua-filters>.
 
 Contributing
 ------------