Update index.md (#30805)

guide/english/python/using-pip/index.md

@@ -67,3 +67,10 @@ For example, we could create the file `requirements.txt` and its content:
      
  Should install all the modules listed on the file.
  
+  If you need to know which packages you have installed (along with their versions) you
+  can use the list command:
+ 
+     pip list
+     
+ This will come in handy when installing packages with prerequisites such as torch or tensorflow.
+