TizenRT: modify README
TizenRT: modify README

I have modified the README file for users who are new to TizenRT.
- Added kconfig-frontends installation method to use menuconfig.

Change-Id: Ibec5798ae47c3d15313ace14c490b488fc2f25cc
Signed-off-by: Junhwan Park <7fee488b6f78fc50b291ffc7d10428788977e877@samsung.com>

JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT