Update readme: pathogenicity instead of ecosystems

README.md

-# Environmental transitions and HGT rates
+# Pathogenicity gains and HGT rates
 
-Code to reproduce the results of the paper, "Horizontal gene transfer rates decrease during transitions to new ecosystems".
+Code to reproduce the results of the paper, "Horizontal gene transfer rates decrease during transitions to new ecosystems", for gain of bacterial pathogenicity instead of gain of new ecosystems.
 
 Jupyter notebooks in `notebooks` directory were run in the order of their numbering. Scripts and other helper functions that they depend on or are mentioned in the notebooks can be found in the `code` directory.