readme: Update toolchain notes
readme: Update toolchain notes

I was quite happy to find out that Keil MDK-ARM actually supports the C99
features I need for this project. The settings for enabling C99 are located
at:

Project -> Options for Target "Target 1" ... -> C/C++ -> C99 Mode

Signed-off-by: Nikolay Dimitrov <dfc921bb2213e22cc8f30fd615839493d2074dee@mail.bg>

picmaster/mikrogui