Add a note about updating plugins in Makefile

Makefile

@@ -91,6 +91,8 @@ bootstrap:
 		go get -u $$tool; \
 	done
 
+# Note: if you have plugins in GOPATH you can update all of them via something like:
+# for i in $(ls | grep vault-plugin-); do cd $i; git remote update; git reset --hard origin/master; dep ensure -update; git add .; git commit; git push; cd ..; done
 update-plugins:
 	grep vault-plugin- vendor/vendor.json | cut -d '"' -f 4 | xargs govendor fetch