Fix setup script in contributing doc (microsoft#1684)

* Fix setup script in contributing documentaion * Add entry file * Fix script error * Address pull request * Updated to highlight keyword * Address comment * thanks myself

Author: lingyv-li

CONTRIBUTING.md

@@ -22,7 +22,7 @@
 ```shell
 git clone https://github.com/microsoft/vscode-python
 cd vscode-python
-yarn install
+yarn install --lock-file
 ```
 
 You may see warnings that ```The engine "vscode" appears to be invalid.```, you can ignore these.

news/3 Code Health/1682.md

@@ -0,0 +1,2 @@
+Change yarn install script to include the keyword `--lock-file`
+(thanks [Lingyu Li](https://github.com/lingyv-li/))