docs/contributing.md: update docs to reflect npm version requirements

Contributing to the extension requires npm v7 or higher, because the
lock file format changed significantly with npm v7.

Change-Id: I486aabc4ccb628628355dc9213281d06811b5b9d
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/331049
Trust: Suzy Mueller <[email protected]>
Run-TryBot: Suzy Mueller <[email protected]>
Reviewed-by: Hyang-Ah Hana Kim <[email protected]>

Author: suzmue

docs/contributing.md

@@ -34,7 +34,7 @@ Please note that extra configuration is required to build and run the [Debug Ada
 
 ### Setup
 
-1) Install [node](https://nodejs.org/en/).
+1) Install [node](https://nodejs.org/en/). Note: make sure that you are using `npm v7` or higher. The file format for `package-lock.json` (changed significantly)[https://docs.npmjs.com/cli/v7/configuring-npm/package-lock-json#file-format] in `npm v7`.
 2) Clone the repository, run `npm install`, and open VS Code:
 
     ```bash