message: Document implicit arguments in the reference
diff: @@ -39,6 +39,7 @@ elaboration, and core language is forthcoming.
  - [Function types](#function-types)
  - [Function literals](#function-literals)
  - [Function applications](#function-applications)
  + - [Implicit parameters](#implicit-parameters)
  - [Records](#records)
  - [Record types](#record-types)
  - [Record literals](#record-literals)
  @@ -726,6 +727,27 @@ let id =
  id Type S32
  ```
  +### Implicit parameters
  +
  +Function parameters can be made *implicit* by prefixing them with `@`:
  +
  +```fathom
  +fun (@A : Type) => A
  +```
  +
  +Implicit parameters will be filled in automatically when they can be inferred by the types of other terms:
  +
  +```fathom
  +let id = fun (@A : Type) (a : A) => a;
  +id false
  +```
  +
  +Implicit parameters can be passed explictly by prefixing the argument with `@`:
  +
  +```fathom
  +id @Bool false
  +```
  +
  ## Records
  Records are types formed out of a combination of other types, each type
  