Added to api.md

docs/src/api.md

@@ -3,6 +3,10 @@
 This is a list of documentation associated with every single **exported** function from `OMOPCDMPathways`.
 There are a few different sections with a brief explanation of what these sections are followed by relevant functions.
 
-```@contents
-Pages = ["api.md"]
-```
+
+## Pre-Processing
+
+This family of functions are dedicated to pre-process the Data.
+
+```@docs
+Period_prior_to_index