Move README.knc to platforms directory.
Update to KNC README for release.

- Add limitations section.
- Reflect default settings in the instructions.
- Remove various notes like things
Update based on Thomas' comments
Merge pull request #476 from sungeunchoi/knc-README-update

KNC README update

- Add limitations section with most recent restrictions
- Reflect default settings in the instructions
- Update instructions for Cray machines
- Remove various notes like things