Import release documentation.

refs: #11262

RELEASE.md

+Upstream
+========
+
+* update version in `configure.in`
+* update first line of `ChangeLog`
+* commit
+* create the tarball:
+
+        git tag -s backupninja-$VERSION
+        ./autogen.sh
+        ./configure
+        make dist
+
+* compare the content of the generated tarball with the content of the
+  previous one
+* `mv backupninja-$VERSION.tar.gz ../tarballs/`
+
+* cleanup: `make distclean`
+
+* sign the release:
+
+        cd ../tarballs
+        gpg --armor --detach-sign backupninja-$VERSION.tar.gz
+
+* upload the generated tarball and detached signature to
+  https://0xacab.org/riseuplabs/backupninja/
+* push master branch and tags
+* announce on the backupninja mailing-list
+
+Debian
+======
+
+        ln -s backupninja-$VERSION.tar.gz backupninja_$VERSION.orig.tar.gz
+        cd ../git
+        git checkout upstream
+        git merge backupninja-$VERSION
+        git checkout debian
+        git-import-orig ../tarballs/backupninja-$VERSION.tar.gz
+        git-dch --auto
+        dch -e
+        git commit debian/changelog -m "Releasing backupninja ($DEBIAN_VERSION) to Debian unstable"
+        git tag -s -m "Releasing backupninja ($DEBIAN_VERSION) to Debian unstable" backupninja_debian/$DEBIAN_VERSION
+        git-buildpackage
+
+* push the debian branch
+* publish the source package somewhere
+* ask someone listed in the @Uploaders@ control field to review and upload
+* push the tag, once uploaded to Debian