Add flags to readme

[Gregory Dudek]

I suggest documenting the flags, and their usage, in the readme file.
They can be found in main.go or via "timeliner -help"

For example:

timeliner -repo /Volumes/XDrive get-all google_photos/[e-mail]

[Matt Holt]

Yeah, will do, after I solidify the CLI more. I might move most of the flags into a config file.