message: rage: Add README note about how to use the feature flags
diff: @@ -140,6 +140,10 @@ Help from new packagers is very welcome.
  ### Feature flags
  +When building with Cargo, you can configure rage using `--no-default-features`
  +and `--features comma,separated,flags` to enable or disable the following
  +feature flags:
  +
  - `mount` enables the `rage-mount` tool, which can mount age-encrypted TAR or
  ZIP archives as read-only. It is currently only usable on Unix systems, as it
  relies on `libfuse`.
  