edit readme

Author: AstroJacobLi

README.md

@@ -14,11 +14,11 @@ pip install -e .
 ```
 
 ## Usage
-
 ```python
 import smplotlib
 ```
-Here's an example figure generated by `smplotlib.demo_plot()` (scientifically it's not that correct, but it's just an example). This figure is inspired by the two-phase ISM figure in Prof. Bruce Draine's [book](https://www.astro.princeton.edu/~draine/book/index.html). 
+Then just use `matplotlib.pyplot` as usual. For a quick start, you can use `smplotlib.demo_plot()` to generate a figure with the default settings. This figure is inspired by the two-phase ISM figure in Prof. Bruce Draine's [book](https://www.astro.princeton.edu/~draine/book/index.html), but the cooling curve doesn't correspond to the actual CR rates and ISRF.
+
 ![example](two_phase.png)
 
 ## Hershey font