Add documentation for sphere input data

test_data/README.md

 # Folder instructions
 
 This directory collects the input files to test the code.
+
+## Instructions
+
+The execution of the original code can be controlled through a set of configuration files that define the characteristics of the problem and affect the type of output. The following sections describe the contents of the configuration files and, to some extent, thos of the output ones, presenting one code in each section.
+
+### cluster
+
+cluster is designed to calculate a complex geometry made up by many spheres. These can be either fully embedded in a larger sphere or separated within the external medium. Sphere compenetration is not accounted for.
+
+TODO: add the description of the cluster configuration files
+
+### sphere
+
+sphere is designed to perform the simplest case calculation, namely the scattering of incident radiation on a single sphere. To perform the calculation, the two following formatted files need to be provided:
+
+TODO: write the the DEDFB documentation
+
+- DSPH
+```
+  SPHERE_NUMBER  MAXIMUM_L_ORDER  POLARIZATION_STATUS  TRANSITION_SHARPNESS_1  TRANSITION_SHARPNESS_2  GEOMETRY
+  STARTING_INC_THETA  INC_THETA_STEP  FINAL_INC_THETA  STARTING_SCA_THETA  SCA_THETA_STEP  FINAL_SCA_THETA
+  STARTING_INC_PHI  INC_PHI_STEP  FINAL_INC_PHI  STARTING_SCA_PHI  SCA_PHI_STEP  FINAL_SCA_PHI
+  WRITE_INTERMEDIATE_FILE
+EOF_CODE
+```
+were the different lines have the following roles:
+1. general configuration of the scattering problem, with some specification of the transition between materials
+2. definition of the elevation angle arrays for the incident and scattered radiation fields
+3. definition of the azimuth angle arrays for the incident and scattered radiation fields
+4. a flag to set whether the intermediate data should be written to output files
+5. an end-of-file code (generally 0)