last-working-dir: add README

plugins/last-working-dir/README.md

+# last-working-dir plugin
+
+Keeps track of the last used working directory and automatically jumps into it
+for new shells, unless:
+
+- The plugin is already loaded.
+- The current `$PWD` is not `$HOME`.
+
+Adds `lwd` function to jump to the last working directory.