README.rst is missing instructions and submodule init fails

1. There seem to be some syntax issues in the README.rst file, GitLab does not render the code-blocks.
2. The instruction git submodule update --init is missing, so I got the error FileNotFoundError: [Errno 2] No such file or directory: './lib/mxgraph/javascript/examples/grapheditor/www/deflate/pako.min.js'
3. The file .gitmodules contains the URL git@github.com:jgraph/mxgraph.git but that requires authentication and results in the error Permission denied (publickey). fatal: Could not read from remote repository. but the public URL of that repository https://github.com/jgraph/mxgraph.git works fine. After it had failed, I replaced that URL in .gitmodules but had to replace it in the (tainted) file .git/config as well to get it working.