Add shutdown message to README.rst (#88)

README.md

@@ -1,3 +1,11 @@
+**WARNING: As previously announced on our communication channels, the Kytos
+project will enter the "shutdown" phase on May 31, 2021. After this date,
+only critical patches (security and core bug fixes) will be accepted, and the
+project will be in "critical-only" mode for another six months (until November
+30, 2021). For more information visit the FAQ at <https://kytos.io/faq>. We'll
+have eternal gratitude to the entire community of developers and users that made
+the project so far.**
+
 # Kytos/UI
 
 This is the web user interface for the [Kytos project](https://kytos.io). For more information