Add instructions on how to egencache overlay

README.md

@@ -6,3 +6,24 @@ necessary to set up a gentoo github mirror based sync system:
 * updates the glsa directory
 * updates herds.xml
 * updates the news directory
+
+## Notes
+
+If you want to sync all overlays additionally to the gentoo repository, you can
+add the file `/etc/paludis/hooks/sync_post/06-sync_overlay_cache.bash` with the
+following content:
+```
+#!/bin/bash
+
+source "${PALUDIS_EBUILD_DIR}/echo_functions.bash"
+source "/etc/paludis/hooks/set_portdir.bash"
+
+if [[ ${TARGET} != gentoo && $(${CAVE} print-repository-metadata --raw-name format --format "%v" ${TARGET}) == e ]] ; then
+	# Number of jobs for egencache, default is number or processors.
+	egencache_jobnum="$(nproc)"
+
+	ebegin "Cache update for repository ${TARGET}"
+	egencache --jobs=${egencache_jobnum} --repo=${TARGET} --update --update-use-local-desc
+	eend $?
+fi
+```