Update README.md

Author: ankit7540

python_module/intensity_calibration/model_rotationalRaman_spectra/README.md

@@ -30,7 +30,13 @@ If using Python3, directly import as
 Available  Functions
 -------------------
 `sumofstate_H2(  T  )`
-`spectra_H2(T, J_{Stokes}, J_{anti-Stokes})`, and  similar for HD and D<sub>2</sub> Here,  temperature T  is in  Kelvin. `J_{Stokes}` and `J_{anti-Stokes})` represents  the maximum rotational J whose intensity is  to be  computed. Intensities are  computed for `J=J_{anti-Stokes},.., J_{Stokes}`. Computed  intensities are relative normalized intensities.
+`spectra_H2(T, J_{Stokes}, J_{anti-Stokes})`, and  similar for HD and D<sub>2</sub> The sum of state or the partition function is defined as <br>
+
+<p align="center">
+  <img   src="https://github.com/ankit7540/RamanSpecCalibration/blob/master/img/partition_function_defn.png" data-canonical-src="https://github.com/ankit7540/RamanSpecCalibration/blob/master/img/partition_function_defn.png" width="581" height="85" />
+</p>
+
+Here,  temperature T  is in  Kelvin. `J_{Stokes}` and `J_{anti-Stokes})` represents  the maximum rotational J whose intensity is  to be  computed. Intensities are  computed for `J=J_{anti-Stokes},.., J_{Stokes}`. Computed  intensities are relative normalized intensities.
 
 Functions can be used directly for computing the sum of states for the three gases for given temperature (for example,  `sumofstate_D2(298)`). When computing spectra the sum of states  are computed within the function (for example, `spectra_H2(298, 6, 6)`).