Added README file to the dirpersist plugin (#7274)

plugins/dirpersist/README.md

+# Dirpersist plugin
+
+This plugin keeps a running tally of the previous 20 unique directories in the $HOME/.zdirs file.  When you cd to a new directory, it is prepended to the beginning of the file.
+
+To use it, add `dirpersist` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... dirpersist)
+```