Added missing information regarding MPI in combination with AppImage

README.md

@@ -11,8 +11,8 @@ support the project by citing our publications given in [REFERENCE.md](REFERENCE
 
 The documentation of PICLas can be found in the [User Guide](https://piclas.readthedocs.io/), where the installation procedure is
 described in [Chapter 2](https://piclas.readthedocs.io/en/latest/userguide/installation.html).
-Pre-compiled executables for Linux can directly be downloaded as AppImage containers from the
-[PICLas release tag assets](https://github.com/piclas-framework/piclas/releases).
+Pre-compiled executables (that only require pre-installed MPI for parallel execution) for Linux can directly be downloaded as
+AppImage containers from the [PICLas release tag assets](https://github.com/piclas-framework/piclas/releases).
 
 ## Regression Testing
 

docs/documentation/userguide/installation.md

@@ -10,6 +10,8 @@ In that case, you can jump directly to the description of the download and insta
 ## AppImage executable download
 
 PICLas and its tools can be installed on a Linux machine without the need of compiling the source code.
+The only requirement is that MPI is pre-installed on the target system and available when running the AppImage executables.
+MPI is not distributed within the AppImage package because of the system-dependent optimizations (e.g. specific InfiniBand settings).
 Currently, PICLas executables are available as *[AppImage](https://appimage.org/)* for Linux.
 Other operating systems, such as Windows or MacOS might be supported in the future.