README: Add usage warning

README.md

@@ -1,5 +1,16 @@
 # PyPSA-Eur-Sec: A Sector-Coupled Open Optimisation Model of the European Energy System
 
+
+
+*WARNING*: This model is under construction and contains serious
+problems that distort the results. See the github repository
+[issues](https://github.com/PyPSA/pypsa-eur-sec/issues) for some of
+the problems (please feel free to help or make suggestions). There is
+neither documentation nor a paper yet, but we hope to have a preprint
+out by summer 2020. We cannot support this model if you choose to use
+it.
+
+
 PyPSA-Eur-Sec builds on the electricity generation and transmission
 model [PyPSA-Eur](https://github.com/PyPSA/pypsa-eur) to add demand
 and supply for the following sectors: transport, space and water
@@ -7,6 +18,18 @@ heating, biomass, industry and industrial feedstocks. This completes
 the energy system and includes all greenhouse gas emitters except
 waste management, agriculture, forestry and land use.
 
+
+PyPSA-Eur-Sec was initially based on the model PyPSA-Eur-Sec described
+in the paper [Synergies of sector coupling and transmission
+reinforcement in a cost-optimised, highly renewable European energy
+system](https://arxiv.org/abs/1801.05290) (2018) but it differs by
+being based on the higher resolution electricity transmission model
+[PyPSA-Eur](https://github.com/PyPSA/pypsa-eur) rather than a
+one-node-per-country model, and by including biomass, industry,
+industrial feedstocks, aviation, shipping, better carbon management,
+carbon capture and usage/sequestration, and gas networks.
+
+
 PyPSA-Eur-Sec includes PyPSA-Eur as a
 [snakemake](https://snakemake.readthedocs.io/en/stable/index.html)
 [subworkflow](https://snakemake.readthedocs.io/en/stable/snakefiles/modularization.html#snakefiles-sub-workflows). PyPSA-Eur-Sec