Remove reference to .NET Foundation support.

[peteraritchie]

MagicMapper/README.md

Line 51 in 534436c

Check out the [getting started guide](https://automapper.readthedocs.io/en/latest/Getting-started.html). When you're done there, the [wiki](https://automapper.readthedocs.io/en/latest/) goes in to the nitty-gritty details. If you have questions, you can post them to [Stack Overflow](https://stackoverflow.com/questions/tagged/automapper)