using rst notes (#182)

Author: choldgraf

docs/index.md

@@ -25,6 +25,7 @@ MyST has the following main features:
 * **[A superset of CommonMark markdown][commonmark]**. Any CommonMark markdown
   (such as Jupyter Notebook markdown) is natively supported by the MyST parser.
 
+You may use MyST markdown **in addition to** using reStructuredText in Sphinx.
 See {doc}`using/intro` to get started.
 
 ```{note}

docs/using/intro.md

@@ -40,6 +40,16 @@ To use the MyST parser in Sphinx, simply add: `extensions = ["myst_parser"]` to
 
 Naturally this site is generated with Sphinx and MyST!
 
+```{admonition} You can use both MyST and reStructuredText
+:class: tip
+
+Activating the MyST parser will simply *enable* parsing markdown files with MyST, and the rST
+parser that ships with Sphinx by default will still work the same way. You can have
+combinations of both markdown and rST files in your documentation, and Sphinx will
+choose the right parser based on each file's extension. Sphinx features
+like cross-references will work just fine between the pages.
+```
+
 ## How does MyST parser relate to Sphinx?
 
 The Sphinx documentation engine supports a number of different input types. By default,