update contributing info about node/npm versions

Author: etpinard

.github/PULL_REQUEST_TEMPLATE.md

@@ -16,6 +16,7 @@ Before opening a pull request, developer should:
 
 - `git rebase` their local branch off the latest `master`,
 - make sure to **not** `git add` the `dist/` folder (the `dist/` is updated only on version bumps),
+- make sure to commit changes to the `package-lock.json` file (if any),
 - write an overview of what the PR attempts to do,
 - select the _Allow edits from maintainers_ option (see this [article](https://help.github.com/articles/allowing-changes-to-a-pull-request-branch-created-from-a-fork/) for more details).
 

CONTRIBUTING.md

@@ -34,9 +34,14 @@ We use the following [labels](https://github.com/plotly/plotly.js/labels) to tra
 #### Prerequisites
 
 - git
-- [node.js](https://nodejs.org/en/). We recommend using node.js v6.x or v4.x (both LTS).
-  Upgrading and managing node versions can be easily done using
-  [`nvm`](https://github.com/creationix/nvm) or its Windows alternatives.
+- [node.js](https://nodejs.org/en/). We recommend using node.js v8.x, but all
+  versions starting from v4 should work.  Upgrading and managing node versions
+  can be easily done using [`nvm`](https://github.com/creationix/nvm) or its
+  Windows alternatives.
+- [`npm`](https://www.npmjs.com/) v5.x and up (which ships by default with
+  node.js v8.x) to ensure that the
+  [`package-lock.json`](https://docs.npmjs.com/files/package-lock.json) file is
+  used and updated correctly.
 
 #### Step 1: Clone the plotly.js repo and install its dependencies