Update README.md (#270)

Author: chahank

README.md

@@ -7,6 +7,12 @@
 
 CLIMADA stands for **CLIM**ate **ADA**ptation and is a probabilistic natural catastrophe impact model, that also calculates averted damage (benefit) thanks to adaptation measures of any kind (from grey to green infrastructure, behavioural, etc.).
 
+CLIMADA is divided into two parts: 
+1. the core [climada_python](https://github.com/CLIMADA-project/climada_python) contains all the modules necessary for the probabilistic impact, the averted damage, uncertainty and forecast calculations. Data for hazard, exposures and impact functions can be obtained from the API. Litpop is included as demo Exposures module, and Tropical cyclones is included as a demo Hazard module. 
+2. the petals [climada_petals](https://github.com/CLIMADA-project/climada_petals) contains all the modules for generating data (e.g., TC_Surge, WildFire, OpenStreeMap, ...). Most development is done here. The petals builds-upon the core and does not work as a stand-alone.
+
+It is recommend for new users to begin with the core (1).
+
 This is the Python (3.8+) version of CLIMADA - please see https://github.com/davidnbresch/climada for backward compatibility (MATLAB).
 
 ## Getting started