Making supported operating systems more clear

Author: benthecarman

doc/release-notes.md

@@ -47,8 +47,9 @@ processing the entire blockchain.
 Compatibility
 ==============
 
-Bitcoin Core is extensively tested on multiple operating systems using
-the Linux kernel, macOS 10.10+, and Windows 7 and newer (Windows XP is not supported).
+Bitcoin Core is supported and extensively tested on operating systems using
+the Linux kernel, macOS 10.10+, and Windows 7 and newer.  It is not recommended
+to use Bitcoin Core on unsupported systems.
 
 Bitcoin Core should also work on most other Unix-like systems but is not
 frequently tested on them.