Update README.md

Author: ppp-one

README.md

@@ -1,5 +1,5 @@
 # mphot
-*mphot* is a Python package to model photometry data from the ground or space.
+*mphot* is a Python package to model photometry for ground or space-based astronomy. Exposure time calculator (ETC) built in.
 
 ![example plots](examples/example-plots.png)
 
@@ -58,4 +58,4 @@ If you find *mphot* useful for your research, please cite [Pedersen et. al 2024]
   year={2024},
   organization={SPIE}
 }
-```
+```