.gitignore: Ignore system configurations created in Mac OS

DS_Store files are automatically created by Mac OS X Finder in browsed directories.
These files contain information about system configuration. If you upload them along with
other files, the files can be misused to obtain information about your computer.

Ref: https://helpx.adobe.com/in/dreamweaver/kb/remove-ds-store-files-mac.html



Signed-off-by: Aayush Gupta <aayushgupta219@gmail.com>
Change-Id: Ie18bd10fe4f254db22c57e7d83207d783dc14b14