Remove obsolete extern crate oxipng advice in lib usage instructions

This syntax hasn't been required since at least Rust's 2018 edition,
except in very specialized scenarios where you want to convince the
compiler that a crate is actually used to guarantee that the
side-effects of its build script (e.g., library linking) are visible,
which do not apply to Oxipng in any case.

README.md

@@ -106,13 +106,11 @@ docker run --rm -v $(pwd):/work ghcr.io/shssoichiro/oxipng -o 4 /work/file.png
 ## Library Usage
 
 Although originally intended to be used as an executable, Oxipng can also be used as a library in
-other Rust projects. To do so, simply add Oxipng as a dependency in your Cargo.toml,
-then `extern crate oxipng` in your project. You should then have access to all of the library
-functions [documented here](https://docs.rs/oxipng). The simplest
-method of usage involves creating an
-[Options struct](https://docs.rs/oxipng/latest/oxipng/struct.Options.html) and
-passing it, along with an input filename, into the
-[optimize function](https://docs.rs/oxipng/latest/oxipng/fn.optimize.html).
+other Rust projects. To do so, simply add Oxipng as a dependency in your Cargo.toml. You should then
+have access to all of the library functions [documented here](https://docs.rs/oxipng). The simplest
+method of usage involves creating an [Options
+struct](https://docs.rs/oxipng/latest/oxipng/struct.Options.html) and passing it, along with an
+input filename, into the [optimize function](https://docs.rs/oxipng/latest/oxipng/fn.optimize.html).
 
 It is recommended to disable the "binary" feature when including Oxipng as a library. Currently, there is
 no simple way to just disable one feature in Cargo, it has to be done by disabling default features