GitLab

This merge request comprises of the following changes:

• Using markdown .md instead of plain text .txt
• Migrating README-icgrep.txt to README-icgrep.md and making its content more readable.
• Separating Parabix and icgrep readme files.
• Creating a README.md file that is easier to read and has all initial information future devs will need.
• Adding badges (ci and Gitter) to README.md file