doc: Update release process

The actual release process quite diverged from what was written here,
also clarify things a bit.

Author: laanwj

doc/release-notes.md

@@ -1,6 +1,21 @@
 (note: this is a temporary file, to be added-to by anybody, and moved to
 release-notes at release time)
 
+Bitcoin Core version *version* is now available from:
+
+  <https://bitcoin.org/bin/bitcoin-core-*version*/>
+
+This is a new major version release, including new features, various bugfixes
+and performance improvements, as well as updated translations.
+
+Please report bugs using the issue tracker at github:
+
+  <https://github.com/bitcoin/bitcoin/issues>
+
+To receive security and update notifications, please subscribe to:
+
+  <https://bitcoincore.org/en/list/announcements/join/>
+
 Notable changes
 ===============