mssql_login documentation added.

documentation/modules/auxiliary/scanner/mssql/mssql_login.md

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+## Vulnerable Application
+
+This module simply queries the MSSQL instance for a specific user/pass (default is sa with blank). 
+
+## Verification Steps
+Example steps in this format (is also in the PR):
+
+1. Start msfconsole
+2. Do: ```use scanner/mssql/mssql_login```
+3. Do: ```set RHOSTS [IP]```
+4. Do: ```run```
+5. You should get a shell.
+
+## Options
+A number of options interesting options without default values exist. They are as follows: 
+
+   Name                 Current Setting  Required  Description
+   ----                 ---------------  --------  -----------
+   PASSWORD                              no        A specific password to authenticate with
+   PASS_FILE                             no        File containing passwords, one per line
+   RHOSTS                                yes       The target host(s),
+   USERPASS_FILE                         no        File containing users and passwords separated by space, one pair per line
+   USER_FILE                             no        File containing usernames, one per line
+
+Options ending in _FILE are used by specifing a file location. For example, specifying the USER_FILE would happend as follows:
+
+```
+msf6 auxiliary(scanner/mssql/mssql_login) > set USER_FILE ./userfile
+```
+
+
+## Scenarios
+Specific demo of using the module that might be useful in a real world scenario.
+```
+msf > use scanner/mssql/mssql_login
+msf (auxiliary(scanner/mssql/mssql_login)) > set RHOSTS 178.33.113.209
+msf (auxiliary(scanner/mssql/mssql_login)) > run 
+
+[*] 59.36.92.188:1433     - 59.36.92.189:1433 - MSSQL - Starting authentication scanner.
+[-] 59.36.92.188:1433     - 59.36.92.189:1433 - LOGIN FAILED: WORKSTATION\sa: (Unable to Connect: The connection with (59.36.92.188:1433) timed out.)
+[*] 59.36.92.188:1433     - Scanned 1 of 1 hosts (100% complete)
+[*] Auxiliary module execution completed
+```