Include exaFOAM benchmarks into README

- The main README.md file (landing page) provides
  now an overview of the benchmark suite
- Description of an individual case is provided in
  the README.md located in the root directory of
  the case. This description is linked within the
  landing page.
- For an intuitive navigation every case from the
  landing page is supplied with a thumbnail image

incompressible/simpleFoam/HPC_motorbike/README.md

+# OpenFoam used version:
+v1912
+
+# rationale
+Three different meshes are proposed, with increasing number of cells. The number of mesh
+cells is increased not through a mesh refinement operation, but by increasing the size of
+the domain.
+Meshes are built as follows:
+- S mesh is built using the motorbike tutorial setup, with the following characteristics:
+    * the background mesh is finer than the one used in the tutorial: generated cells have
+      a 0.4 meter side, instead of 1 meter
+    * the snappyHexMeshDict is the same as in the tutorial
+- M and L are obtained using the mirroMesh functionality starting from the S mesh so that
+  M is 2xS and L is 2xM
+- Mesh calculation is performed using snappyHexMesh in parallel, with 16 tasks
+
+# how to build the S,M,L meshes
+- load you OpenFOAM env
+- get into the desired mesh dir
+- launch ./AllmeshX script, with X=S,M,L
+
+# how to run the case
+- the cases can be run in the same folder where the meshes have been computed
+- edit the decomposeParDict to change the number of tasks
+- launch ./Allrun
+
+# info on S,M,L meshes
+- S: 8.5 mln cells
+- M: 17 mln cells
+- L: 34 mln cells
+
+# notes
+- consider an average execution time of about 35 minutes using 16 cores fro S mesh.
+- M and L meshes will be done in few more minutes thanks to the mirroMesh function.
+- if you like to activate the checkMesh option please consider more time as overall execution time
+- Other larger meshes can be built using the same approach