Add blurb to docs about updating ext/node when updating Deno version (#494)

Author: pRizz

DEVELOPERS.md

@@ -64,6 +64,8 @@ and run `orb` in the base directory to enter the virtual machine.
   https://github.com/denoland/deno/blob/v1.30.3/Cargo.toml)
 - Open the `Cargo.toml` at the root of of this repo and modify all `deno_*`
   modules to match to the selected tag of Deno.
+- Update the contents of `ext/node/` to match the contents of
+  `deno@selected-tag/ext/node`.
 
 ## How to use Dev Container