Update NEWS.md

Added new math-jax config option to NEWS.md

NEWS.md

@@ -1,5 +1,9 @@
 # CHANGES IN bookdown VERSION 0.41
 
+- New `mathjax-config` config option to bs4_book and gitbook to control MathJax config string (thanks, @bwu62, #1472). Currently tested and supported settings:
+  - If empty, defaults to original `TeX-MML-AM_CHTML` which renders all equations in common HTML.
+  - If set to `TeX-AMS-MML_HTMLorMML` renders equations in HTML + CSS (which may look nicer for some equations).
+  - If set to `TeX-MML-AM_SVG` renders equations in SVG.
 
 # CHANGES IN bookdown VERSION 0.40