Add information on doing a database migration

Author: golf1052

CONTRIBUTING.md

@@ -1,5 +1,7 @@
 # Contributing
 
+See more documentation in the `docs` folder.
+
 ## Building
 
 ### Requirements

docs/database.md

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+# Database
+
+We use [Entity Framework Core](https://docs.microsoft.com/en-us/ef/core/) with SQLite as our [database provider](https://docs.microsoft.com/en-us/ef/core/providers/sqlite/?tabs=dotnet-core-cli).
+
+## Updating the Database Model
+
+1. Add or remove properties in the model classes.
+2. Open the Package Manager Console in Visual Studio
+3. Ensure the default project is selected as `Signal-Windows.Lib` as this is where the DbContext classes are.
+4. Run `Add-Migration <migration name> -Context <DbContext class>`. Typically `migration name` is just the number of the migration (m4, m5, m6, etc.). `DbContext class` is the class name of the DB we're migrating. Most migrations are to `SignalDBContext` but `LibsignalDBContext` is also valid.
+5. Debug the app to ensure the migration runs successfully. Migrations are triggered [here](https://github.com/signal-csharp/Signal-Windows/blob/55598a6bdf57ce22f48fc18bc587b257122115a0/Signal-Windows/App.xaml.cs#L60-L61) as soon as the app is started up.