README: add caution of menuconfig in Docker env
"make menuconfig" applies only in Kconfig-frontend installed env,
not in Docker env. But beginner of TizenRT could not know it.
Let's guide it in first README.
@@ -58,6 +58,9 @@ After basic configuration by [3. Configuration](#3-configuration), you can custo
./dbuild.sh menuconfig
```
+**Note**: In Docker environment, ```make menuconfig``` command from other README files should be replaced with this command.
+ It applies only in manual setup build environment.
+
This command might require ```sudo``` for root permission.
To run Docker without ```sudo```, refer to https://docs.docker.com/install/linux/linux-postinstall/.
