better document recommended node version (#10411)

doc/Developing.md

@@ -53,7 +53,8 @@ For Windows instructions [click here](#building-on-windows).
 ## Prerequisites
 
  - Node.js `>= 12.14.1`.
-   - Preferably, **use** Node's [Active LTS version](https://nodejs.org/en/about/releases/).
+   - For now, **use** Node 12.
+   - Node 14 is supported, Node 16 _should_ work, but you might need to run `yarn --ignore-engines` to get around some arbitrary constraints.
    - If you are interested in Theia's VS Code Extension support then you should use a Node version at least compatible with the one included in the version of Electron used by [VS Code](https://github.com/microsoft/vscode).
  - [Yarn package manager](https://yarnpkg.com/en/docs/install)  `>= 1.7.0` **AND** `< 2.x.x`.
  - git (If you would like to use the Git-extension too, you will need to have git version 2.11.0 or higher.)