added text to the progress chapter

Showing Progress

 [ built-in progress meter, progress callback ]

 For historical and traditional reasons, libcurl has a built-in progress meter

 that can be switched on and then makes it presents a progress meter in your

 terminal.

 Switch on the progress meter by, oddly enough, set CURLOPT_NOPROGRESS to

 FALSE. This option is set to TRUE by default.

 For most applications however, the built-in progress meter is useless and

 what instead is interesting is the ability to specify a progress

 callback. The function pointer you pass to libcurl will then be called on

 irregular intervals with information about the current transfer.

 Set the progress callback by using CURLOPT_PROGRESSFUNCTION. And pass a

 pointer to a function that matches this prototype:

        int progress_callback(void *clientp,

                              double dltotal,

                              double dlnow,

                              double ultotal,

                              double ulnow);

 If any of the input arguments is unknown, a 0 will be passed. The first

 argument, the 'clientp' is the pointer you pass to libcurl with

 CURLOPT_PROGRESSDATA. libcurl won't touch it.

libcurl with C++