Added a ReadOnly.md which can be used as the service-alert if an Alert.md is not present AND if the gallery is in Read-Only Mode

1) Tested that if Alert.md is not present, and it is not read-only, no banner is shown
2) Tested that if Alert.md is not present, and it is in Read-Only mode, Read-Only mode banner is shown
3) Tested that if Alert.md is present, and it is not Read-Only mode, Alert banner is shown
4) Tested that if Alert.md is present, and it is Read-Only mode, still Alert banner is shown
Merge pull request #2253 from NuGet/daravind/ReadOnlyAlert

Added a ReadOnly.md which can be used as the service-alert if an Alert.m...