add command z

reference/static/commands/tools/z.md

@@ -0,0 +1,18 @@
+z
+---
+
+`z` is a jump-around command that learns and navigates to your most frequently and recently (termed 'frecent') used directories. You can add it to your shell by cloning https://github.com/rupa/z and following the instructions in the README.
+
+~~~ bash
+$ z foo
+~~~
+
+---
+
+### Useful Options / Examples
+
+You can cd to most frecent dir matching foo, then bar as follow.
+
+~~~ bash
+$ z foo bar
+~~~