Posts a generic feed notification:

## Parameters
* **isImportant**: Makes the notification flash on the screen.
* **showInBrief**: Makes the notification appear in the "Pause Menu > Info/Brief > Notifications" section.
## Return value
The notification handle.
## Examples
```lua
AddTextEntry("MyFeedMessage", "Hello " .. GetPlayerName(PlayerId()) .. ".")
BeginTextCommandThefeedPost("MyFeedMessage")
EndTextCommandThefeedPostTicker(true, true)