Added a script for generating status badges in README.md.

The script scans through git submodules in the superproject and generates
a table with CI status badges as well as other relevant links for all the
submodules. Multiple tables can be created for different git branches.

The script inserts the generated content between two marker Markdown comments
in README.md, removing any previous content between them. This allows the
script to be simply rerun to update the list of submodules.

This commit also updates README.md with the generated tables for master and
develop.