Manual: missing steps in section 4.6

[raffaello-camoriano]

I am implementing new functionalities in GURLS++. To do so, I followed the steps in Section 4.6 of the GURLS manual. The steps are correct, but I had to make some other modifications in order to integrate my functionalities and be able to build.

The steps are lested below:

1) In gurls.h, the following instruction has to be added in the proper group of includes:

#include "gurls++/<NEWSUBCLASSFILE>.h"

2) In .h, just before

/**
 * \ingroup Exceptions

The next lines shoud be added:

template <typename T>
class <NEWSUBCLASS>;

[andreaschiappacasse]

Thank you, with the latest commit the manual is now updated to include these informations.