Add release notes for v0.30.6

Summary:
As per title.

Depends on D17282.

Test Plan: Read it.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17280

Author: Fabcien

doc/release-notes.md

@@ -7,3 +7,12 @@ Bitcoin ABC version 0.30.6 is now available from:
 This release includes the following features and fixes:
  - A new `getinfo` RPC has been added to retrieve basic information about the
    node.
+ - A bug only affecting nodes with avalanche disabled has been fixed. This bug
+   could cause the node to crash under certain circumstances. Avalanche is
+   enabled by default and disabling the feature is highly discouraged.
+ - The MacOS release is now built with Chronik support, which can be enabled
+   with the `-chronik` option just like the other platforms.
+ - Starting with this version, support for MacOS 10.15 Catalina has been
+   removed. The release build requires MacOS 11.0 (Big Sur) or above.
+ - The MacOS .dmg file will no longer be distributed and is replaced by a .zip
+   archive. To install, just drag the application into the Applications folder.