Update the pin layout table and consolidate the RPi readme files.

 - Changes in the pin layout table to describe how to connect the NRF24L01 pins to different boards
 - Remove duplicate readme information in the RPi readme file and point to the default RPi/RF24/readme.md file.
Select a standard pin layout for the examples.

Choose a standard pin layout for all the examples based on the pin connection table in the readme files.
Different pin layout in the example files could lead to not working examples.
Fix wrong pin layout
Merge pull request #21 from reixd/examples

Select a standard pin layout for the examples and update README files.