Update database instructions to work in .NET Core 3.0

In the 3.0 SDK `dotnet ef` needs to be installed. This updates the instructions/tool config to do so.

More info: https://docs.microsoft.com/en-us/ef/core/miscellaneous/cli/dotnet

README.md

@@ -61,6 +61,7 @@ You can also run the samples in Docker (see below).
 
 ```
 dotnet restore
+dotnet tool restore
 dotnet ef database update -c catalogcontext -p ../Infrastructure/Infrastructure.csproj -s Web.csproj
 dotnet ef database update -c appidentitydbcontext -p ../Infrastructure/Infrastructure.csproj -s Web.csproj
 ```