CONTRIBUTING.md is outdated

[Mingun]

First of all, there are no clear instructions how to test changes in the derive macro. What steps is necessary to run all testsuites?

• There is no folder test_suite/deps, which is required to run tests

• If I try to run tests on stable rust from the repository root, I get the error:

  d:\Projects\serde>cargo test
    Compiling serde_test_suite v0.0.0 (D:\Projects\serde\test_suite)
  error[E0658]: non-ascii idents are not fully supported
    --> test_suite\tests\test_gen.rs:271:9
      |
  271 |         σ: f64,
      |         ^
      |
      = note: see issue #55467 <https://github.com/rust-lang/rust/issues/55467> for more information
  
  error: aborting due to previous error
  
  For more information about this error, try `rustc --explain E0658`.
  error: could not compile `serde_test_suite`
  
  To learn more, run the command again with --verbose.
  
  d:\Projects\serde>
  

  This error is quite strange, because the referenced issue is closed.

• I was only able to ran tests when I supply the "unstable" feature flag, which requires the nightly rust:

  cargo +nightly test --features unstable
  

  Is it possible to run tests on stable?