Add a badge to install ADIOS2 via conda-forge.
By default, conda-forge packages install without MPI. The user can
control variants via:

- `conda install -c conda-forge adios2` default, nompi variant
- `conda install -c conda-forge adios2=*=mpi*` any mpi-compatible implementation
- `conda install -c conda-forge adios2=*=mpi_mpich*` pick mpich variant.
  Same result as `conda install -c conda-forge mpich adios2=*=mpi*` .
- `conda install -c conda-forge adios2=1.13=nompi*` explicitly forbid MPI.
  Usually not necessary, as the build number offset causes `nompi` to be preferred unless MPI is explicitly requested.