Add information about purpose of branches to README

Need to extend table with purpose of branches in README:

Branch Name	Purpose of the Branch
main	Starting point of the project without CI.
3-basic-ci-pipeline	Basic CI pipeline has been added.
4-extending-the-pipeline	CI pipeline has been extended with common use cases.
5-removing-redundancies	Redundancies have been removed form CI pipeline.
6-optimizing-the-pipeline	CI pipeline has been optimized to let run more CI jobs in parallel.
7-using-includes	Structure of CI pipeline has been adapted to use multiple files.