Add IPYNB_IGNORE_CSS to README.md (#57)

IPYNB_IGNORE_CSS was added in b319a45 to fix #48. This
documents how to use it.

README.md

@@ -163,7 +163,8 @@ while trying to make it the most general and useful out of the box as possible,
 Jupyter Notebook is based on bootstrap so you probably will need your theme to be based on that it if you want the html and css to render nicely.
 
 I try to inject only the necessary CSS, removing Jupyter's bootstrap but fixes are needed in some cases,
-if you find this issues I recommend looking at how my theme fixes them.
+if you find this issues I recommend looking at how my theme fixes them. You can suppress the inclusion of CSS entirely by setting
+`IPYNB_IGNORE_CSS=True` in `pelicanconf.py`. 
 
 
 ## Options
@@ -179,3 +180,4 @@ when the summary creation should stop, this is usefull to generate valid/shorter
 `default = [('div', ('class', 'input')), ('div', ('class', 'output'))]`
 - `IPYNB_EXTEND_STOP_SUMMARY_TAGS`: list of tuples to extend the default `IPYNB_STOP_SUMMARY_TAGS`
 - `IGNORE_FILES = ['.ipynb_checkpoints']`: prevents pelican from trying to parse notebook checkpoint files
+- `IPYNB_IGNORE_CSS = True`: do not include the notebook CSS in the generated output