Explain "-e" install. (See posit-dev#461)

Author: mccalluc

README.md

@@ -84,4 +84,14 @@ Or get a full list of helpers with just:
 make
 ```
 
+Typically, when developing new features for Shiny, you'll want to try them out in an application.
+In a **separate** application directory, use can use `-e` to reference your local checkout of `py-shiny`:
+
+```sh
+# Rather than
+#   pip install shiny
+# run:
+pip install -e ../py-shiny --config-settings editable_mode=compat
+```
+
 See the [docs README](docs/README.md) for instructions on building the documentation locally.