Suggest/Talk about --all-targets and feature gating

[Mark-Simulacrum]

See rust-lang/rust#51208 (comment); copying:

So --all-targets seems to work great. Feature handling is harder, but since rustfix supports incremental migration that's fine, for crates with non-mutually exclusive features, cargo +nightly fix --prepare-for 2018 -- --all-targets --no-default-features and cargo +nightly fix --prepare-for 2018 --allow-dirty -- --all-targets --all-features should be sufficient to migrate.

If exclusive features are present users need to run with --feature blah and then --features baz. For the alpha release that seems sufficient; we might want rustfix to automatically do this with some flag, but I don't think that's very feasible due to exclusionary feature groups.

We should update the migration docs.

[Mark-Simulacrum]

@nikomatsakis suggested that we also ask users to provide feedback about the feature patterns beyond all/none that they use.

[alexcrichton]

One question that I think would be worth considering is whether to enable this sort of behavior by default as well, as

cargo +nightly fix --prepare-for 2018 --allow-dirty -- --all-targets --all-features 

is pretty wordy!

[Mark-Simulacrum]

--all-features is also not technically enough, as you might have features which enable/disable functionality when enabled/disabled, e.g. something like a std feature which is on by default but when disabled you'll see alloc or core imports, for example

[Centril]

Closing since #70 rendered this moot.