Document requirement for cURL / shell_exec
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
dregad/source-integration,make-all/source-integration,dregad/source-integration,mantisbt-plugins/source-integration,mantisbt-plugins/source-integration,mantisbt-plugins/source-integration,dregad/source-integration,make-all/source-integration,make-all/source-integration