[docs] Add link to gfortran wiki for binaries

[François-Xavier Coudert]

>> And I'll note that the current Wiki page is relatively weak. For example:
>>
>> . Please change the Wiki page to read GNU/Linux instead of Linux to
>> follow the rules mandated by the FSF.
>>
>> . "detailed installation instructions" point to an empty page.
>>
>> . The basic web usability rule of not naming links "here" or "see here"
>> are violated.

Please feel free to commit these changes and any other you might find
useful. That's what Wikis are for, after all.

FX