Document requirement for cURL / shell_exec

The _SourceBitBucket_ and _SourceGithub_ plugins rely on cURL to perform
OAuth requests and make API calls.

When the PHP cURL extension is not installed, the plugins use shell_exec
to call curl; if that fails or system calls are not authorized, then it
won't be possible to complete the request.

Document the cURL requirement in the README file.

Fixes #214

Author: dregad

README.md

@@ -51,6 +51,11 @@ integration framework and API.
 The Source Integration framework requires [MantisBT](http://www.mantisbt.org/)
 version **2.0.0** or higher.
 
+Additionally, the _SourceBitBucket_ and _SourceGithub_ plugins
+require the [PHP Curl extension](http://php.net/book.curl), or the ability to
+execute system calls ([shell_exec](http://php.net/function.shell-exec)).
+
+
 ## Installation
 
 ### Compatibility