Top Level Readme Improvements

[danewalton]

Two parts

• [x] Break down the Repo Structure to Repo Structure and Samples Callout. Explain why we have two repos. Explain if you want to integrate into already existing project, clone this repo. If wanting samples, clone the samples.
• [x] Make the samples callout the first thing below the "why"

AKA

# Azure IoT middleware for FreeRTOS
  --stuff--
## Samples
  --stuff--
## Repo Structure
  --stuff--