Update README.md - add Security section content from wiki + update header format

Author: bimimicah

README.md

@@ -8,7 +8,7 @@ Original Author: Nathan Neulinger (nneul@umr.edu)
 
 ![high-level overview diagram of mod_authnz_external](/documentation/mod_authnz_external%20overview.png)
 
-### Support Matrix
+## Support Matrix ##
 
 <table><thead><th>Apache Version</th><th>mod_authnz_external Version</th><th>mod_authz_unixgroup Version</th><th>Supported?</th></thead><tbody>
 <tr><td rowspan='2'> Apache 2.4 </td><td> <b>mod_authnz_external 3.3.x</b> </td><td> <b>mod_authz_unixgroup 1.2.x</b> </td><td> Yes </td></tr>
@@ -20,13 +20,16 @@ Original Author: Nathan Neulinger (nneul@umr.edu)
 
 Older versions are provided on an as-is basis in this repo's [branch list](https://github.com/phokz/mod-auth-external/branches/all).
 
-### Security Considerations
+## Security Considerations ##
+mod_authnz_external can be used to quickly construct secure, reliable authentication systems.  It can also be mis-used to quickly open gaping holes in your security.  Read the documentation, and use with extreme caution.
+
+Use of this module requires development of an external authentication program or a hardcoded internal function.  These are typically very simple programs, but there are more ways to screw up your security by doing them badly than we could possibly list. See the file [AUTHENTICATORS](../blob/master/mod_authnz_external/AUTHENTICATORS) or the [How to Write an External Authenticator or Group Checker](AuthHowTo) wiki document for more information on implementing authenticators.
 
 Older versions of mod_auth_external would by default pass logins and passwords into the authentication module using environment variables. This is insecure on some versions of Unix where the contents of environment variables are visible on a 'ps -e' command. In more recent versions, the default is to use a pipe to pass sensitive data. This is secure on all versions of Unix, and is recommended in all installations.
 
 People using mod_auth*_external with pwauth to authenticate from system password databases should be aware of the [innate security risks](http://code.google.com/p/pwauth/wiki/Risks) involved in doing this.
 
-### mod_authz_unixgroup
+## mod_authz_unixgroup ##
 
 This repo is also the home of mod_authz_unixgroup, a unix group access control module for the [Apache HTTP Server](http://httpd.apache.org).