Update the readme file

The readme file in this repository has last been updated 15 years ago and does not accurately reflect more recent versions of the frontend. Possible improvements include:

• Converting the file to UTF-8 (the most widely used text encoding)
• Adding build instructions for Cabal
• Translating the text to English for consistency, since the rest of the repository is kept in English (comments, issues, etc)
• Updating references to cymake to curry-frontend
• Converting the file to Markdown
• Renaming it to README(.md/.txt), so that GitLab displays it on the repository page