add link in blog post

Author: dwrensha

blog/_posts/2023-09-04-0.18-release.md

@@ -138,7 +138,8 @@ which can sometimes be a problem for microcontroller targets and kernel programm
 (See [this issue](https://github.com/capnproto/capnproto-rust/issues/221)
 for some discussion.)
 
-Starting with version 0.18, the `capnp` crate now has an `alloc` Cargo feature,
+Starting with version 0.18, the `capnp` crate now has an
+[`alloc` Cargo feature](https://github.com/capnproto/capnproto-rust/blob/82aa8a2163b5eadaa83aa2716ce781fc78107c2c/capnp/Cargo.toml#L26),
 which can be disabled to remove the `alloc` dependency.
 
 A side benefit of this change is that now error handling in capnproto-rust