Adds '.DS_Store' to .gitignore.
Adds '.DS_Store' to .gitignore.

swisscom/docs-appcloud-book,cloudfoundry/docs-book-cloudfoundry,cloudfoundry/docs-book-cloudfoundry,cloudfoundry/docs-book-cloudfoundry,cloudfoundry/docs-book-cloudfoundry,swisscom/docs-appcloud-book,swisscom/docs-appcloud-book,swisscom/docs-appcloud-book