Add and document DISTKEY var for 'make dist'

Makefile.in

@@ -149,6 +149,9 @@ dist:
 		if [ -n "${DISTGPGID}" ]; then \
 			gpg --sign --detach-sig --armor --local-user ${DISTGPGID} $$tarball; \
 		fi; \
+		if [ -n "${DISTKEY}" ]; then \
+			../vendor/signify-osx/signify -S -s "${DISTKEY}" -m $$tarball; \
+		fi; \
 	done
 
 

portmgr/ReleaseProcess.md

@@ -149,6 +149,11 @@ details, see [HOW TO SPECIFY A USER ID in gpg(1)][gpg-user-id] for details.
 
     make dist DISTVER=2.0.0 DISTGPGID=<handle>@macports.org
 
+A signify(1) signature should also be generated for the use of selfupdate
+over http. Specify the path to the secret key in `DISTKEY`.
+
+    make dist DISTVER=2.0.0 DISTKEY=/path/to/macports/macports-base-2024.sec
+
 These tarballs and the checksums are uploaded to the
 https://distfiles.macports.org/MacPorts/ directory. At present, this must be
 done with the help of the infrastructure team.