Document minimum node v12 & v14 versions in README. (sucrase issue pull

sveltejs#1000).

See my comment

sveltejs#1000 (comment)

references

>Node.js 14.13.0 added support for named CJS exports, see #35249 and this change has also been backported to Node.js v12.20.0.

nodejs/node#32137 (comment)

and

>Yep, it was fixed after downloading 14.13.1, thank you

nodejs/node#35249 (comment)

README.md

@@ -8,6 +8,14 @@ Please see [the documentation](https://kit.svelte.dev/docs) for information abou
 
 ## Developing
 
+Ensure that you have a recent version of node:
+
+```
+node -v
+```
+
+For node v12 you should have at least `v12.12.0` and for node v14 at least `v14.13.1`. 
+
 This monorepo uses [pnpm](https://pnpm.js.org/en/). Install it...
 
 ```bash