remind users to run deno install after adding imports to a deno.json … (#2469)

Author: thisisjofrank

runtime/reference/jsx.md

@@ -70,6 +70,13 @@ To use the newer JSX runtime transform change the compiler options in your
 }
 ```
 
+:::note install dependencies
+
+When manually adding imports to your `deno.json` file, remember to run
+`deno install` to download and cache the dependencies.
+
+:::
+
 Behind the scenes the `jsxImportSource` setting will always append a
 `/jsx-runtime` to the import specifier.