docs(readme): Add difference between pip freeze

README.rst

@@ -35,4 +35,9 @@ Example
     $ pipreqs /home/project/location
     Successfuly saved requirements file in: /home/project/location/requirements.txt
  
+Why not pip freeze?
+-------------------
 
+- ``pip freeze`` only saves the packages that are installed with ``pip install`` in your environment. 
+- pip freeze saves all packages in the environment including those that you don't use in your current project. (if you don't have virtualenv)
+- and sometimes you just need to create requiremetns.txt for a new project without installing modules.