Better highlight DDBEGIN/DDEND in the documentation

[fred-wang]

DDBEGIN/DDEND are not mentioned at all in the command line help, and only mentioned briefly as a reduction hint on the README.md file at https://github.com/MozillaSecurity/lithium/#hints

The command line help actually suggests to read using-for-firefox.md which again mentions DDBEGIN/DDEND briefly at https://github.com/MozillaSecurity/lithium/blob/master/src/lithium/docs/using-for-firefox.md#preparing-the-testcase

I believe this DDBEGIN/DDEND feature is a nice trick to help reduction and I believe it deserves more visibility. What documenting it in the command line help or main README instead of the using-for-firefox doc? At Igalia we have successfully been using lithium to reduce testcases in other browsers, including Chromium and WebKit (thanks BTW!) and one may easily overlook the "using-for-firefox" help (actually, the command line help should probably refer to something else than using-for-firefox.md but I guess that's a separate issue).