The latter is now the preferred URL for visiting cmake.org with a
browser. Convert using the shell code:
git ls-files -z | xargs -0 sed -i 's|http://www\.cmake|https://cmake|g'
Rename 'Readme.txt' to 'README.rst', format the documentation as
reStructuredText, and modernize the content. Also add a new
'CONTRIBUTING.rst' explaining the preferred entry path for new
contributors.