add README file with (adjusted) schematic

Author: Sabine-van-Dijk

applications/GeoMechanicsApplication/tests/test_element_lab/test_triaxial/README.md

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+# Triaxial test
+
+This test is a triaxial test with a prescribed displacement on a Mohr-Coulomb model. It mimics a lab test, where soil properties such as the cohesion ($c$) and the friction angle (ϕ) are defined.
+In the lab this is performed on a cylindric volume of soil, where an increasing pressure is applied on the top of the cylinder. In the model test, the cylinder is emulated by two 2 axisymmetric elements. 
+
+A schematic overview of the model is displayed in the figure below:
+
+![MeshStructure](schematic.svg)
+
+## Setup
+
+The test is performed with the following conditions:
+
+- Constraints:
+    - The displacement in the bottom nodes (5, 8, 9) is fixed in the Y direction.
+    - The displacement in the symmetry axis (i.e. the left nodes 1, 3, 5) is fixed in the X direction.
+    - The displacement of the top nodes (1, 2, 6) is prescribed and moves linearly from y = 0 on the top to y = -0.2 at t = 1.
+- Material:
+    - The material is described by the Mohr-Coulomb model with the following parameters:
+        - Poisson ratio = 0.25,
+        - Young's modulus = 20000 $kN/m^2$,
+        - Cohesion = 2.0 $kN/m^2$,
+        - Friction angle = 25.0 $\degree$,
+        - Dilatancy angle = 2.0 $\degree$.
+- Conditions:
+  - An initial uniform stress field is applied with a value of -100 $kN/m^2$ in all directions.
+  - A lateral load is applied with a value of 100 $kN/m^2$ to the right side.
+
+## Assertions
+For this regression test, the outcomes of the simulation for the displacement, the Cauchy stress and the engineering strain at t = 1 are asserted.