update instruction for PR'ing Node.js

Node.js currently has automation in place
to update its dependencies. We can probably
relax the suggestion here.

Author: cjihrig

README.md

@@ -33,4 +33,6 @@ etc. You can verify that this step worked by checking
 [here](https://github.com/libuv/libuv/commit/07955ed3737cc59bc4d586b34222669ca87de755).
 7. Create a new release on Github for the tag at
 <https://github.com/libuv/libuv/releases/new>.
-8. Make a pull request to nodejs/node to update the version of libuv.
+8. Optionally, make a pull request to nodejs/node to update the version of libuv.
+At the time of writing, the Node.js project has automation in place to update its
+libuv dependency.