Improved clarity in rpi.md
Improved clarity in rpi.md

Thought process behind some decisions. Fundamental reason is because most users tend to straight away move to docs without exploring about the project much so there are various painpoints that user might get stuck: 

* Users like me who are Raspberry Pi 3 have no clue whether it would work on our system or not. It works on my RPI3 and you guys have tested on RPI 1 & 2. Adding this disclaimer in the start assuages user. 
* User get's to know that he/she has to build opensourceFrameworks in the last. Most users tend not to read the entire guide in one go, generally they follow step by step. Therefore moved the step ahead. 
* Added links for `getrepos.sh` file. Since user has no idea that he/she has to use the getrepos.sh, provided the steps. The user could have cloned FootFall but he/she already does in the next steps therefore copying works for now. 
* Deleted repeated steps for update and upgrade. 
* Fixed whitespace error

Also `sudo raspi-update` doesn't work. I googled and there this is command called rpi-update.
WatershedArts/Footfall,WatershedArts/Footfall,WatershedArts/Footfall,WatershedArts/Footfall