Make limitations of the adapter more evident in the documentation

[IshaanDesai]

Recently in a presentation, users of this adapter pointed out that the limitations of the adapter are not explicitly mentioned in the documentation. This led the users to believe that the adapter supports general simulations in the way that FEniCS and preCICE do. This is not true; the adapter has several limitations, which have been explained in detail, in the following issues:

• https://github.com/precice/fenics-adapter/issues/149
• https://github.com/precice/fenics-adapter/issues/65
• https://github.com/precice/fenics-adapter/issues/7

These limitations should be mentioned more prominently in the documentation, so that users know what the adapter is not capable of doing out of the box.