Add Maintenance Disclaimer to README (#71)

This PR adds a disclaimer to the README that the repository is no longer
being actively developed at the moment by the protocol team, although we
expect to resume development some time in the near future.

README.md

@@ -1,5 +1,6 @@
-# Safe{Core} Protocol Specification
+**:warning: This repository is not actively developed at the moment. :warning:**
 
+# Safe{Core} Protocol Specification
 
 See also:
 - [Protocol Whitepaper](./whitepaper.pdf)