add NormalizedReadRatio equation details in doc

docs/source/methods.md

@@ -26,6 +26,16 @@ Methods
 
  - **8 - MultiQC:** Generates QC report
 
+## Read ratio computation
+
+The following equation is used to compute the read ratio
+
+$$NormalizedRead_{Ratio} = \log2\left(\frac{\frac{N_{\ aDNA\ bp \ aligned \ genome1}}{size_{genome2} }}{\frac{N_{ \ aDNA \ bp \ aligned \ genome2}}{size_{genome2}}}\right)$$
+
+An increase of the NormalizedReadRatio by 1 is equivalent to a two fold increase of the number of base pairs.
+
+Note that only basepairs originating from PMD carrying reads (as identified by PMDtools) and aligned to a reference genome with an identity greater than the predefined threshold, are taken into account for the calculation of the NormalizedReadRatio.
+
 
  ## Example pipeline graph