added description how to merge two repositories

Update README.md

See merge request !5

- merged sections on git submodules and git subrepo

See merge request !4

added section on Handling multiple repositories (e.g. by using git-subrepo)

See merge request !3

adding howto to split subfolder into a new repository

See merge request !1

Update README.md on installation of git bfs

See merge request !2

  git-subrepo)

  git-subrepo)