Update Contribution Guide to require node v12+

Installing the development branch runs into an issue if node version is
older than 12.

CONTRIBUTING.md

@@ -69,7 +69,7 @@ You can launch a binder with the latest JupyterLab master to test something (thi
 
 ### Installing Node.js and jlpm
 
-Building JupyterLab from its GitHub source code requires Node.js.
+Building JupyterLab from its GitHub source code requires Node.js. The development version requires Node.js version 12+.
 
 If you use `conda`, you can get it with:
 
@@ -85,6 +85,12 @@ brew install node
 
 You can also use the installer from the [Node.js](https://nodejs.org) website.
 
+To check which version of Node.js is installed:
+
+```bash
+node -v
+```
+
 ## Installing JupyterLab
 
 JupyterLab requires Jupyter Notebook version 4.3 or later.