message: Add TRACE usage to docs
  Summary: As title.
diff: @@ -3,6 +3,8 @@ id: usage
  title: Usage
  ---
  +## Basic Usage
  +
  To use ReDex, first build your app and find the APK for it. Then the simplest
  invocation is:
  ```
  @@ -119,10 +121,23 @@ optional arguments:
  From here, you may want to read the [configuration guide](config.md) and details
  about the [passes](passes.md).
  +The result `output.apk` should be smaller and faster than the
  +input.
  +
  +## Tracing
  +
  If you want some statistics about each pass, you can turn on tracing:
  ```
  export TRACE=1
  ```
  -
  -The result `output.apk` should be smaller and faster than the
  -input. Enjoy!
  +More specifically, tracing has [categories](https://github.com/facebook/redex/blob/c5d5651b8b3ae9fda7b3305de9f55e1b82077a2d/libredex/Trace.h#L20)
  +and levels. For a `TRACE(X, Y, msg)` statement in the code to be logged,
  +category `X` must have level `Y` or higher:
  +```
  +export TRACE=X:1,Y:2,Z:3
  +```
  +The output of tracing can also be redirected to a file with the `TRACEFILE`
  +variable:
  +```
  +export TRACEFILE=/path/to/trace.txt
  +```
  