update readme on mathjax v3

Author: archmoj

README.md

@@ -87,11 +87,19 @@ While non-minified source files may contain characters outside UTF-8, it is reco
 
 > Please note that as of v2 the "plotly-latest" outputs (e.g. https://cdn.plot.ly/plotly-latest.min.js) will no longer be updated on the CDN, and will stay at the last v1 patch v1.58.5. Therefore, to use the CDN with plotly.js v2 and higher, you must specify an exact plotly.js version.
 
-To support MathJax, you need to load version two of MathJax e.g. `v2.7.5` files from CDN or npm.
+To support MathJax, you could load version two or three of MathJax e.g. `v2.7.5` files from CDN or npm.
 ```html
 <script src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/MathJax.js?config=TeX-AMS-MML_SVG.js"></script>
 ```
 
+```html
+<script src="https://cdn.jsdelivr.net/npm/[email protected]/es5/tex-svg.js"></script>
+```
+
+> When using MathJax version 3, it is also possibly to use `chtml` output on the other parts of the page in addition to `svg` output for the plotly graph.
+Please refer to `devtools/test_dashboard/index-mathjax3chtml.html` to see an example.
+
+
 ## Bundles
 There are two kinds of plotly.js bundles:
 1. Complete and partial official bundles that are distributed to `npm` and the `CDN`, described in [the dist README](https://github.com/plotly/plotly.js/blob/master/dist/README.md).