Improved the vagrant example of a beowulf cluster bringing out the comments and adding the configuration files

examples/vagrant/beowulf cluster/README.md

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+# Beowulf cluster example
+
+## Requirements:
+
+- VirtualBox
+- A host-only virtual network (`VirtualBox Host-Only Ethernet Adapter`, address space `192.168.56.0/24`)
+- An SSH key pair (files `id_ed25519` and `id_ed25519.pub`, create it with the command below)
+
+  ```sh
+  ssh-keygen -f 'id_ed25519' -N '' -C 'controller'
+  ```
+
+- An SSH config file ([`ssh_config.txt`][ssh_config.txt])
+- A list of hosts for MPICH ([`mpi_hosts.txt`][mpi_hosts.txt]) containing the IP addresses of the workers
+
+When up, execute the command below and enjoy:
+
+```sh
+vagrant ssh -c 'mpiexec -f mpi_hosts -n 3 hostname'
+```
+
+<!-- internal references -->
+[mpi_hosts.txt]: mpi_hosts.txt
+[ssh_config.txt]: ssh_config.txt