Major README changes

[oconnor663]

• Rewrite the introduction. Include a more intelligent comparison to gclient and git submodules.
• Updated GIF.
• More about how override works.
• More about recursion.
• Split some sections off into separate docs.

[oconnor663]

Thoughts about gclient:

• No reup.
• All your dependencies are laid out as full repos.
• If a subrepo is dirty as in "contains unstaged changes" then sync will complain, but if it's dirty as in "pointed at a commit other than the one you synced" then sync will silently pave it.
• Git and SVN only. status and diff seem to be broken for git as well.
• Top level projects have .gclient files, whereas dependency projects have DEPS files in a different format.

[oconnor663]

Thoughts about repo:

• Only works with git.
• Integrated more closely with server-side stuff. (Need to play with this more.)

[oconnor663]

Need to take a look at git-annex.

[oconnor663]

Also need to update Configuration with the latest flags.

[oconnor663]

Figure out which Linux distros can't execute binaries installed by pip without adding something to PATH.

[oconnor663]

Figure out who needs to worry about pip vs pip3.

[oconnor663]

Most of the above is done, but we need to add an advanced section, and factor stuff out. I'm going to add this to the 1.0 milestone. It doesn't necessarily need to be done before we tag 1.0, but probably before we start writing about it.