Move documentation files for packaging preparation

Move doc/coder.txt into doc/source/ where Sphinx will expect it,
doc/LICENSE to the top-level directory renamed to LICENSE.rst for
clarity on its format, and similarly rename README to README.rst as
well.

diff --git a/doc/LICENSE b/LICENSE.rst
similarity index 100%
rename from doc/LICENSE
rename to LICENSE.rst

diff --git a/README b/README.rst
similarity index 100%
rename from README
rename to README.rst

diff --git a/doc/coder.txt b/doc/source/coder.rst
similarity index 100%
rename from doc/coder.txt
rename to doc/source/coder.rst