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'

Reported-by: Radovan Bast

Suggested-by: Petr Bena <benapetr@gmail.com>

Use the name "Kitware" rather than "Kitware, Inc." to simplify the text.
Provide a CMake-specific URL.

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.