Fix for Content/ContentTypeReaderManager.cs to allow type resolution from calling project.

Whilst creating a starter kit to test custom content pipeline use and serialisation/deserialisation (from the MS CustomModelClass Sample) I encountered an issue where types from the calling project (custom types) could not be de-serialised through Monogame.  This was simply because the ContentTypeReaderManager could only resolve from the MonoGame project alone

This fix allows custom types in projects to be resolved in content reader type resolution from a client solution

Will also post the StarterKit demonstrating its use