Doc: add documentation covering usage of the new --cfg flags

docs/book/src/usage.md

@@ -1,3 +1,29 @@
 # Usage
 
 This section is intended for users who want to install Marker and run provided lints.
+
+## Controlling Marker Lints
+Once your crate is configured to run some lints, it quickly becomes important to control the lint levels within your
+code. Marker provides the ability to use normal lint control attributes like `#[allow(...)]` `#[deny(...)]` and others
+to control the behavior of marker lints.
+
+### On nightly
+Marker provides this functionality via the `#[register_tool()]` attribute currently available in
+[nightly](https://github.com/rust-lang/rust/issues/66079). If your crate is compiled using nightly, then controlling
+lints is as simple as placing `#![feature(register_tool)]` and `#![register_tool(marker)]` at the top of your crate
+`lib.rs` or `mod.rs` file. You can then use all of the normal lint attributes you might usually use, on marker provided
+lints, like `#[allow(marker::my_lint)]`.
+
+### On Stable
+If your project isn't on nightly, you can still use the `register_tool` attribute, you'll just need to add some extra
+guards around it. Marker provides config arguments to rust during the lint passes, so that your linted code can
+conditionally apply attributes only during the marker run.
+
+Specifically marker passes `--cfg=marker` and a `--cfg=marker="my_crate"` flag for each lint crate. This means that you
+can use `#[cfg_attr(marker, foo)]` to conditionally apply the `foo` attribute only during lint runs. This can be used
+to leverage the fact that lints run using nightly, without requiring the linted crate to compile using nightly. To do
+this, add a `#![cfg_attr(marker, feature(register_tool))]` and `#![cfg_attr(marker, register_tool(marker))]` attributes
+to the top of your crate file, then you can apply lint level attributes like `#[cfg_attr(marker, allow(marker::foo))]`
+to control your marker lints. Additionally, if there are attributes you only want active when marker is aware of
+certain lint crates, you can conditionally apply your attribute with something like
+`#[cfg_attr(marker = "my_crate", allow(marker::foo))]`.