Add a infor that F3 are EOL

README.md

 # INAV - navigation capable flight controller
 
+## F3 based flight controllers
+
+> STM32 F3 flight controllers like Omnibus F3 or SP Racing F3 are deprecated and soon they will reach the end of support in INAV. If you are still using F3 boards, please migrate to F4 or F7.
+
 ![INAV](http://static.rcgroups.net/forums/attachments/6/1/0/3/7/6/a9088858-102-inav.png)
 ![Travis CI status](https://travis-ci.org/iNavFlight/inav.svg?branch=master)