Improve the README regarding the restrictions and supported preCICE features

[Fujikawas]

According to the guideline for adapters, there are some infos missing in the README.md file (or other documentation files), such as:

• application background and/or nature of coupling (e.g., surface vs. volume coupling; transient or steady-state; ...)
• what data can be read and written for a coupling
• how to configure (including the coupling data, if available)
• geometry assumptions and limitations, e.g., number of dimensions, assumptions on out-of-plane axis for lower dimensions, or support for only a single or also multiple patches.
• which preCICE features are supported or not supported. Curated list including:
  • implicit coupling
  • mesh connectivity
  • data initialization
  • subcycling: dt = min(dt_solver, dt_precice)
  • multiple meshes
• restrictions on solver features

These infos can be added to make the applicable scenarios for this adapter more clear.