Significant changes to README.md and M5Stack.h to benefit new users

Today I added text to README and M5Stack.h to help new users become acquainted with the purpose of this library. (Also updates to reflect the EOL status of versions of this product).

The first time I encountered this library, it took me a while just to figure out its capabilities and purpose. Let me save the next guy some time. M5 products are great but I think new users get lost fast in the "documentation by example" especially if they don't know what's there. This pull request changes only comments and markdown, and contains no changes to actual code.