Update build instructions

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Document how to specify Gravatar default image

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
TortoiseGitMerge: Document the missing command line options

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>