Add documentation

The code needs more documentation. This also depends on the status of #6.

• Document the class PreciceAdapter (doxygen or similar) see !23 (merged)
• Document how the code has to be used. Maybe use mkdocs?
• Document installation process (see #24 (closed))
• Add documentation of CouplingManager and explain what "reverse" means in this context. Do we have a better name than "reverse"? See also !8 (merged)
• Add some documentation about tests. Currently we use preCICE 1.6.1, but we should migrate to preCICE v2 soon
  • Current release of DuMuX + preCICE
  • Current development of DuMuX + preCICE
  • Code has been migrated to preCICE v2