README.md is not up-to-date: Cannot Compile with guide

[yezhizhen]

stylo/README.md

Lines 11 to 28 in 8c57436

	## Building Servo against your local Stylo
	Assuming your local `servo` and `stylo` directories are siblings, you can build `servo` against `stylo` by adding the following to `servo/Cargo.toml`:
	```toml
	[patch."https://github.com/servo/stylo.git"]
	derive_common = { path = "../stylo/derive_common" }
	malloc_size_of = { path = "../stylo/malloc_size_of" }
	selectors = { path = "../stylo/selectors" }
	servo_arc = { path = "../stylo/servo_arc" }
	servo_atoms = { path = "../stylo/atoms" }
	size_of_test = { path = "../stylo/size_of_test" }
	static_prefs = { path = "../stylo/style_static_prefs" }
	style_config = { path = "../stylo/style_config" }
	style_derive = { path = "../stylo/style_derive" }
	style = { path = "../stylo/style" }
	style_traits = { path = "../stylo/style_traits" }
	```

This won't compile as ``derive_common` etc. does not exist.

The guide in https://github.com/servo/servo/blob/4f8d816385a5837844a3986cda392bb6c0464fe6/Cargo.toml is correct.

cc @xiaochengh