hint that checkout has to be done first

[SergNikitin]

Hey @Swatinem, thank you for this awesome project!

Disclaimer: I'm not very experienced either with GitHub actions or rust, so this might be an incorrect change -- feel free to ignore the PR if so.

As far as my experience with this action goes, it does not seem to do anything if the repo gets checked out after the cache setup. Placing the actions/checkout step before the rust-cache step seems to solve the problem though. So

# no caching
- uses: actions-rs/toolchain@v1
  with:
    profile: minimal
    toolchain: stable
- uses: Swatinem/rust-cache@v1
- uses: actions/checkout@v2 # checkout at the end

# caching works
- uses: actions/checkout@v2 # checkout at the top
- uses: actions-rs/toolchain@v1
  with:
    profile: minimal
    toolchain: stable
- uses: Swatinem/rust-cache@v1

It might be a common practice in github actions community to always stick actions/checkout at the top of the yaml file, but for someone less experienced like me it may seem intuitive to set up caches first, do everything else after - that's what I did, and had to pay for it with a couple of hours of debugging time.

This change adds the checkout step to the readme example explicitly so people like myself who cluelessly copy-paste examples have less things to trip over :)