Include format specifiers in help text

Previously, the format specifiers were only included in the source
repository README and the source code; this change adds the specifiers
to the program help text as well.

I looked at the QCommandLineOption API and there does not seem to be a
way to specify a subset of allowed strings such that they are
automatically included in the help text by QCommandLineParser - the
canonical way is just to include the options in the matching help text
as I have done in this commit.

src/main.cpp

@@ -41,7 +41,7 @@ int main(int argc, char *argv[])
                                   QCoreApplication::translate("main", "Hz"));
     parser.addOption(rateOption);
     QCommandLineOption formatOption(QStringList() << "f" << "format",
-                                  QCoreApplication::translate("main", "Set file format."),
+                                  QCoreApplication::translate("main", "Set file format, options: cfile/cf32/fc32, cf64/fc64, cs32/sc32/c32, cs16/sc16/c16, cs8/sc8/c8, cu8/uc8, f32, f64, s16, s8, u8, sigmf-meta/sigmf-data."),
                                   QCoreApplication::translate("main", "fmt"));
     parser.addOption(formatOption);