Add symbol-addition to the how-to for new features

[ssbr]

I don't know if I'm holding it wrong, but without these steps, I get the following error, for example:

error[E0425]: cannot find value `manually_drop_attr` in module `sym`                                                                                                                                                [47/1826]
   --> compiler/rustc_feature/src/active.rs:436:14
    |
436 |     (active, manually_drop_attr, "1.64.0", Some(100344), None),
    |              ^^^^^^^^^^^^^^^^^^ help: a constant with a similar name exists: `manually_drop`
    |
   ::: /usr/local/google/home/devinj/local_programming/rust-compiler/compiler/rustc_span/src/symbol.rs:23:1
    |
23  | symbols! {
    | -------- similarly named constant `manually_drop` defined here

[JohnTitor]

Thanks!