* Several, small documentation improvements
- Re-organize main repo README
- Minor clean-ups to Python package-specific readme files
- Clean-up to Unity Inference Engine page
- Update to the docs README
- Added a specific cross-platform section in ML-Agents Overview to amplify Barracuda
- Updated the links in Limitations.md to point to the specific subsections
- Cleaned up the Designing a Learning Environment page. Added an intro paragraph.
- Updated the installation guide to specifically call out local installation
- A few minor formatting, spelling errors fixed.