Add initial at_persistence documentation

Author: jhart-r7

documentation/modules/exploit/multi/local/at_persistence.md

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+## Vulnerable Application
+
+This module executes a metasploit payload utilizing `at(1)` to execute jobs at a specific time.  It should work out of the box
+with any UNIX-like operating system with `atd` running.  In the case of OS X, the `atrun` service must be launched:
+
+```
+sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.atrun.plist
+```
+
+## Verification Steps
+
+  1. Start msfconsole
+  2. Exploit a box via whatever method
+  3. Do: `use exploit/multi/local/at_persistence`
+  4. Do: `set session #`
+  5. Do: `set target #`
+  6. `exploit`
+
+
+## Options
+
+  **TIMING**
+
+  Controls the time value passed to `at(1)`
+
+  **PATH**
+
+  If set, uses this value as the path on the remote system to store the payload.  If unset, uses `mktemp`.
+
+## Scenarios
+
+TBD