Add documentation for post/windows/gather/hashdump

documentation/modules/post/windows/gather/hashdump.md

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+The post/gather/hashdump module functions similarly to Meterpreter's built-in hashdump command.
+Having this feature as a post module allows it to be used in different penetration testing
+scenarios.
+
+
+## Vulnerable Application
+
+---
+
+To be able to use post/gather/hash_dump, you must meet these requirements:
+
+* You are on a Meterpreter type session.
+* Target is a Windows platform.
+* Execute it under the context of a high privilege account, such as SYSTEM.
+
+## Verification Steps
+
+---
+
+Please see Overview for usage.
+
+## Scenarios
+
+---
+
+**Upgrading to Meterpreter**
+
+To be able to use this module, a Meterpreter session is needed. To upgrade to this, the easiest
+way is to use the post/multi/manage/shell_to_meterpreter module. Or, you can try:
+
+1. Use the exploit/multi/script/web_delivery module.
+2. Manually generate a Meterpreter executable, upload it, and execute it.
+
+**High Privilege Account**
+
+Before using post/gather/hashdump, there is a possibility you need to escalate your privileges.
+There are a few common options to consider:
+
+* Using a local exploit module. Or use Local Exploit Suggester, which automatically informs you
+  which exploits might be suitable for the remote target.
+* getsystem.
+* Stolen passwords.
+
+**Hashdump From Multiple Sessions**
+
+One major advantage of having hashdump as a post module is you can run against multiple hosts
+easily. To learn how, refer to Overview for usage.