Document tips for using Grpc.Tools

There are some tips for using Grpc.Tools that would help people if they were more widely known.

For example:

Import protos with wildcards:
```xml
<Protobuf Include="**/*.proto" GrpcServices="Server" />
```

Use ProtoRoot:
```xml
<Protobuf Include="**/*.proto" GrpcServices="Server">
  <ProtoRoot>Protos</ProtoRoot>
</Protobuf>
```